fix(transaction): resolve conditional references before execution

[angela-helios]

Closes #459

Conditional references (Type?query) passed through into storage verbatim, where nothing can search or resolve them — Synthea bundles name practitioners, organizations, and locations this way on every entry, so the #448 dataset ingested with hundreds of unresolvable references.

Per the transaction processing rules they now resolve against the server's content in a pre-pass before anything executes:

• exactly one match → the reference is rewritten to Type/id;
• zero or several matches → the bundle is rejected with a clear message (they are atomic, and silently storing junk was the old behavior).

Distinct references resolve once (bundles repeat the same location heavily) and each lookup searches with count=2 — enough to prove non-uniqueness. The pre-pass runs before write validation so validation sees the rewritten content. batch_handler/process_transaction gain a SearchProvider bound, which every backend and the route-level bound already satisfy.

Scope correction on the issue

The issue's other half — urn:uuid fullUrl rewriting — turned out to be a false alarm: replaying both a minimal transaction and a real Synthea bundle against a clean server shows references correctly rewritten. The dangling urns in the original report came from my #448 $bulk-submit NDJSON (raw bundle-entry resources extracted verbatim), not from the transaction path. Noted on #459.

Tests

Three new integration tests in batch_conformance (unique match rewritten into storage; zero matches reject; ambiguous match rejects); full helios-rest suite green (batch_conformance 42, search_integration 101, and the rest).

[codecov]

Codecov Report

❌ Patch coverage is 98.14815% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/rest/src/handlers/batch.rs	98.14%	2 Missing ⚠️

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