test(model-access): qualify the pinned client and budgeted live Vertex calls

[Brad-Edwards]

Parent design and full capability tracker: #681. Requirement: PLAT-202. Split out of #2124 (M07).

Requirements

• PLAT-202 — Per-Range LLM Access Management

Why this exists

#2124 (M07) implements and hardens the Vertex invocation and usage adapter with tests that exercise real HTTP serialization, streaming, accounting and error framing. Two acceptance elements of M07 require live qualification evidence that cannot be produced from the code checkout alone and are tracked here so #2124 delivers the code hardening without a hand-authored fixture standing in for real evidence:

• The architecture guidance is explicit that "a successful synthetic HTTP fixture alone is insufficient" and that qualification "must use sanitized requests captured from the exact pinned client and installed-settings contract, not hand-authored approximations" (see docs/architecture/model-access/vertex-adapter-preflight-2124.md).
• A separately budgeted live positive call is required for every enabled project/alias before that target is activated.

Scope (live evidence only)

• Capture a sanitized real wire corpus from the exact pinned released client during an ordinary multi-turn coding session: startup, model listing if used, count, non-stream JSON, SSE, useful local tool use/results, both logical aliases, each configured project, default headers/body features, budget edges, provider errors, malformed/truncated usage, slow/disconnected clients and unsupported-feature rejection. Corpus entries carry the client/config version metadata that produced them and contain no prompts, credentials or provider tokens.
• Confirm the pinned client's installed-settings contract admits the qualified Messages subset with no manual client or guest change (the default configuration must work out of the box and must not need participant-side repair).
• Run one separately budgeted live positive call for every enabled project/alias before activation. Record project/model/region, client/dependency/image digests, price/quota/retention prerequisites and safe outcome metadata in the protected evidence surface. Do not commit prompts, provider diagnostics, credentials or private operational identifiers.
• Feed the qualified GCP slice forward per the delivery ledger; keep each future provider/tool profile's evidence separate.

Boundaries

• The adapter code, closed contracts, accounting behavior, token caching, error normalization and synthetic (version-tagged) broker fixtures are delivered in feat(model-access): implement the Vertex invocation and usage adapter #2124.
• Independent deployed IAM / network / revocation boundary proof is owned by test(gcp): prove model-access identity and revocation boundaries #2127 (M10); this issue is the M07 client-compatibility and budgeted-live-call evidence.
• Local mocks, rendered manifests and a successful count are not live invocation proof.

This issue references the existing Ground Control requirement; it does not declare PLAT-202 implemented or tested. Reconcile actual implementation/test traceability through Ground Control when work lands.

Blocked by #2124.