fix: strictSchema "log" should warn (not throw) on unknown format

[chatman-media]

Fixes #2538.

What issue does this pull request resolve?

With strictSchema: "log", an unknown format in a schema throws an exception instead of logging a warning. Per the documented option values, "log" should log a warning while only true should throw.

const ajv = new Ajv({strictSchema: "log"})
ajv.compile({type: "string", format: "unknown"})
// Expected: logs `unknown format "unknown" ...`
// Actual:   throws `Error: unknown format "unknown" ignored in schema at path ...`

Root cause

In lib/vocabularies/format/format.ts, unknownFormat() only special-cased opts.strictSchema === false (warn) and threw for every other value. Since "log" is not === false, it fell through to throw. As a side effect, strictSchema: false also emitted a warning, whereas it should be silent ("ignore all strict schema violations").

Fix

Replace the hand-rolled branch with the shared checkStrictMode helper (lib/compile/util.ts), which is already used throughout the codebase for the true | false | "log" tri-state:

• true → throw
• "log" → logger.warn
• false → silent (no-op)

This also makes the message consistent with every other strict-mode violation (prefixed with strict mode:).

Tests

Added spec/issues/2538_strict_schema_log_unknown_format.spec.ts covering all three modes. The "log" and false cases fail on master and pass with this change; true still throws. Existing spec/options/unknownFormats.spec.ts and the strict-mode suites remain green, and the full JSON Schema conformance suite passes.