[Client] Extend conformance client.php with scenario context env vars

Context

The conformance runner ships per-scenario context via --context JSON which it passes to the client command. The current tests/Conformance/client.php only reads MCP_CONFORMANCE_SCENARIO and the server URL — it ignores client_id, client_secret, private_key_pem, signing_algorithm, idp_id_token, idp_issuer, idp_token_endpoint, idp_client_id. Without these, auth scenarios cannot exercise the SDK even after auth flows are implemented.

Scope

• Read context from the env var the runner sets (verify exact name; commonly MCP_CONFORMANCE_CONTEXT as JSON).
• For each scenario, wire context into Client\Builder:
  • auth/pre-registration → setOAuthClient(client_id, client_secret).
  • auth/client-credentials-basic → setOAuthClient(...) + force client_credentials grant.
  • auth/client-credentials-jwt → setOAuthClientWithKey(client_id, private_key_pem, signing_algorithm).
  • auth/cross-app-access-complete-flow → supply IdP id_token + IdP discovery params.
• Land this before or in parallel with auth flow implementation so each PR can be validated against conformance immediately.

Conformance scenarios unblocked

Prerequisite for all auth scenarios requiring context.

Dependencies

Independent — can land first (harness-only).

Acceptance

• tests/Conformance/client.php updated.
• Documented context keys in the test file's docblock.

Start with tests/Conformance/client.php and inspect the conformance runner to verify the exact context environment variable and JSON keys. Read the available Client\Builder methods, then trace the listed auth scenarios to wire their context; done means the file documents the keys and auth scenarios receive the required context for conformance validation.