Add remaining provider boundary and ambiguous-commit tests

[ian-pascoe]

Problem

The review identified two durable provider contracts not yet protected by focused tests:

• PostgreSQL ambiguous COMMIT recovery when the socket disappears immediately before/after COMMIT;
• S3 acceptance of an object exactly at MAX_AUTHORITY_GENERATION_BYTES (the suite covers only limit + 1 rejection).

Acceptance criteria

• Fault-injected PostgreSQL tests prove one generation/receipt/activity and correct replay/error outcome across both ambiguous COMMIT windows.
• S3 test proves exact-limit success and limit-plus-one rejection without an SDK request.
• Tests are deterministic, isolated, and safe under the full suite.

Evidence

Review anchors: packages/core/src/storage/sql/authority.ts around ambiguous commit recovery (~2197) and packages/core/test/storage-s3-authority.test.ts around size limits (~241).