fix(core): preserve default retry match when overriding retry options - #18295

Pull Request Checklist

• Have you added new tests to prevent regressions?
• If a documentation update is necessary, have you opened a PR to the documentation repository?
• Did you update the typescript typings accordingly (if applicable)?
• Does the description below contain a link to an existing issue (Closes #[issue]) or a description of the issue you are solving?
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Description of Changes

Closes #18259.

Sequelize's default retry policy is deliberately conservative:

retry: { max: 5, match: ['SQLITE_BUSY: database is locked'] }

Constructor options are shallow-merged, so supplying any retry block replaces the whole object. A config that only tunes backoff therefore loses the default match:

new Sequelize({ dialect, retry: { max: 5, backoffBase: 1000, backoffExponent: 1.5 } }).options.retry;
// before: { max: 5, backoffBase: 1000, backoffExponent: 1.5 }   // no `match`

In retry-as-promised, an empty/absent match means retry on every error
(options.match.length === 0 || options.match.some(...)). So tuning backoff silently flips the
policy from "retry one transient error" to "retry everything", including permanent ones: a losing
findOrCreate that throws UniqueConstraintError is then retried max times with full exponential
backoff (the issue reports ~8s of wasted waiting and a pinned pooled connection) before failing,
where it should fail on the first attempt.

This merges the user's retry onto the default instead of replacing it, so a partial override keeps
the default match while still letting callers override any field, including match: [] to opt into
retry-all explicitly. The merge is placed after the ...persistedSequelizeOptions spread in the
constructor's option normalization (packages/core/src/sequelize-typescript.ts).

Tests (packages/core/test/unit/configuration.test.ts, no DB): the default policy is kept when no
retry is given; the default match (and max) survive a partial override; an explicit match
still overrides. Failing before the change, passing after; the full core unit suite stays green
(2263 passing).

List of Breaking Changes

None intended, but one observable difference is worth calling out: code that passed a partial retry
object without match was (likely unintentionally) getting retry-on-every-error; it now retains the
default match: ['SQLITE_BUSY: database is locked']. Callers who actually want to retry all errors
should set match: [] explicitly.

Summary by CodeRabbit

• Bug Fixes
  • Fixed retry option merging so custom retry settings no longer overwrite default retry behavior unexpectedly.
  • Default retry limits and match rules are preserved when only backoff-related fields are customized.
  • Explicit retry match values (including an empty match) now correctly take precedence over defaults.
• Tests
  • Added unit coverage to verify retry behavior across default, partial override, and explicit match scenarios.