feat(bigtable): wire Diverter on Client and route Open* via TableShim - #20256

…20258)

## Summary

Follow-up to #20257 (protoRowToRow), which now has its sole consumer.
Swaps `TableShim`'s session backend from the classic `TableAPI` shape
(row string + `bigtable.Row`) to the internal
`session.TableAPI` shape (proto-native
`*btpb.SessionReadRow{Request,Response}` +
`*btpb.SessionMutateRow{Request,Response}`).

`TableShim` now owns the proto ↔ public-types translation so the
`internal/session` package can stay proto-native.

## Behavior

**`ReadRow`** — parses `ReadOption` using the classic `makeReadSettings`
shape (so filter + full-read-stats callback plumbing stays in one
place), builds a `*btpb.SessionReadRowRequest`, calls
`session.ReadRow`, feeds any `resp.Stats` through the
`WithFullReadStats` callback, and converts `resp.Row` via
`protoRowToRow`.

**`Apply`** — conditional mutations (`CheckAndMutateRow`) always route
to classic since the session vRPC has no `CheckAndMutateRow`
equivalent. Non-conditional mutations build a
`*btpb.SessionMutateRowRequest` and call `session.MutateRow`.

**`useSession()`** — nil-safe on both `session` and `diverter`, so
callers can wire a `TableShim` with `nil` session (as
`buildDivertible` does in #20256) and every routing decision falls
through to classic.

**`ReadRows` / `SampleRowKeys` / `ApplyBulk` / `ApplyReadModifyWrite`
always classic** — no session equivalent in the vRPC.

## API-visible signature change

`NewTableShim`'s `session` parameter type becomes `session.TableAPI` (an
internal-package interface). Existing in-repo callers pass `nil` for
session today; nil is the zero value of any interface, so the change
is source-compatible for those. External callers writing tests can
mock `session.TableAPI` directly — the new test file has an example
(`mockSessionTable`).

## Tests

Replaces the `mockTableAPI`-as-session pattern with a proto-native
`mockSessionTable`. New coverage:

- `TestTableShim_ReadRow_RoutesByDiverter` — classic when
  `SessionLoad=0.0`; session when `SessionLoad=1.0`; classic fallback
  when session is nil even with `SessionLoad=1.0`; session error
  propagation (no automatic fallback to classic on failure).
- `TestTableShim_Apply_ConditionalAlwaysClassic` — pins that
  conditional mutations bypass the session path.
- `TestTableShim_Apply_NonConditionalRoutesByDiverter` — pins that
  non-conditional mutations follow the diverter.
- `TestTableShim_ReadRows_AlwaysClassic`,
  `TestTableShim_SampleRowKeys_AlwaysClassic`,
  `TestTableShim_ApplyBulk_AlwaysClassic`,
  `TestTableShim_ApplyReadModifyWrite_AlwaysClassic` — pin the
  no-session-equivalent methods.
- `TestTableShim_NilSession_AllMethodsFallBackToClassic` — the
  classic-only wiring path (what #20256's `buildDivertible` will use
  until the session backend lands).
- `TestTableShim_SessionErrorNotRetriedOnClassic` — session-side
  failures surface as-is instead of silently falling back.

## Test plan

- [x] `go build ./...` clean
- [x] `go vet ./...` clean
- [x] `go test ./bigtable/ -run 'TestTableShim|TestProtoRowToRow'
-count=1 -v` — all pass
- [x] Live sandbox smoke against `autonomous-mote-782 / sushanb-uc1` —
classic path via `client.Open(...).Apply/ReadRow` still succeeds
end-to-end (session backend not exercised on this branch since it isn't
wired yet)

## Depends on

- #20257 (merged as `1297143a4f`) — `protoRowToRow` helper.

## Follows

Consumed by a future PR that wires an actual `session.TableAPI`
implementation
from the `internal/session` package into `Client.buildDivertible` (see
#20256 for the `buildDivertible` shape).

🤖 I have created a release *beep* *boop*
---


##
[1.52.0](bigtable/v1.51.0...bigtable/v1.52.0)
(2026-08-03)


### Features

* **bigtable:** Add AFE picker (Simple / LeastInFlight / LeastLatency)
([#20204](#20204))
([bcbf714](bcbf714))
* **bigtable:** Add ClientConfig.DisableSession to opt out of session
backend
([#20297](#20297))
([7ee5e44](7ee5e44))
* **bigtable:** Add getClientConfigDirectAccessChecker for session pools
([#20209](#20209))
([3b8d30a](3b8d30a))
* **bigtable:** Add NoOpChannelPrimer for session channel pools
([#20208](#20208))
([d055a8a](d055a8a))
* **bigtable:** Add per-AFE sessionList for the two-tier session pool
([#20224](#20224))
([dbf0c3f](dbf0c3f))
* **bigtable:** Add protoRowToRow conversion helper for TableShim
([#20257](#20257))
([1297143](1297143))
* **bigtable:** Add Session debug surface (observability fields +
methods)
([#20211](#20211))
([d8d3e16](d8d3e16))
* **bigtable:** Add Session lifecycle (Start, Close, ForceClose,
readLoop, heartBeatLoop)
([#20215](#20215))
([b9e53c6](b9e53c6))
* **bigtable:** Add Session struct + state machine
([#20117](#20117))
([09acbb3](09acbb3))
* **bigtable:** Add session.Config.EnableDebug to gate sessionz debug
state
([#20247](#20247))
([ce74c31](ce74c31))
* **bigtable:** Add SessionClient + SessionTable + lazyPool
([#20228](#20228))
([ab2c96c](ab2c96c))
* **bigtable:** Add SessionPoolImpl (two-tier pool + scaling + debug)
([#20225](#20225))
([683eda8](683eda8))
* **bigtable:** Rename session pool display to
<resource-id>-<PERM>
([#20248](#20248))
([35e146e](35e146e))
* **bigtable:** Route Client.Open()-returned *Table through the Diverter
([#20273](#20273))
([2b81c7d](2b81c7d))
* **bigtable:** State-based classification for abnormal session close
([#20243](#20243))
([f2905b7](f2905b7))
* **bigtable:** TableShim fallback to classic on session UNIMPLEMENTED
([#20269](#20269))
([36540af](36540af))
* **bigtable:** TTL-on-idle cache for per-resource session.TableAPI
([#20263](#20263))
([00b2a49](00b2a49))
* **bigtable:** Wire Diverter on Client and route Open* via TableShim
([#20256](#20256))
([b32fbd7](b32fbd7))


### Bug Fixes

* **bigtable:** AFE picker latency signal — subtract poolWait and
compute TransportLatency = wire − backend at source
([#20281](#20281))
([bb8c4d5](bb8c4d5))
* **bigtable:** Guard NewStream OnFinish against grpc-go double-fire
([#20295](#20295))
([b51da29](b51da29))
* **bigtable:** Real per-resource pool teardown on sessionTable.Close +
cache close-race gate
([#20264](#20264))
([599aea9](599aea9))
* **bigtable:** Session.durations / session.uptime — set explicit
histogram bucket boundaries
([#20276](#20276))
([97eee22](97eee22))
* **bigtable:** SessionTableHandle self-heals across cache eviction
([#20296](#20296))
([0dd98cd](0dd98cd))
* **bigtable:** Translate ctx errors to gRPC status on session vRPC
([#20299](#20299))
([0f3b2a5](0f3b2a5))
* **bigtable:** Treat PingAndWarm NotFound as a successful prime
([#20219](#20219))
([a1557ad](a1557ad))


### Performance Improvements

* **bigtable:** Delete periodic Tick loop; sizing is event-driven
([#20285](#20285))
([2c096bd](2c096bd))
* **bigtable:** Drop pick_lost_race debug tag from CheckoutSession hot
path
([#20280](#20280))
([bd0e400](bd0e400))

---
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