fix(healthcare): support MCP tool discovery

[dinakarjs]

The healthcare mock interpreted tools/list as tools/call, returning an unknown-tool error and leaving upstream drift unchecked. Route by method and list all four EHR tools with the approved catalog descriptions and MCP inputSchema values. Existing call handlers remain unchanged; unsupported methods return -32601.

Follow-up to #134, limited to healthcare/server/mock_mcp_server.py. Upstream definitions are stored independently of the gateway catalog so a change remains detectable.

Validation:

• Live HTTP listing matches all four catalog definitions and handler names, preserving the request ID. All four calls match existing handlers; unknown tool/method and wrong-path cases pass.
• Published cmcp-runtime 0.7.0 in development mode: standard scenario allows all four calls; high-risk and contraindication allow the first three and deny the final write with POLICY_DENY. Each closed session produces a signed TRACE record with no drift or catalog exceptions. No tools discovery incomplete warning.
• Negative control: changed ehr.patient_record_lookup’s upstream description while retaining the approved catalog. Runtime logs UPSTREAM_CATALOG_DRIFT and blocks with HTTP 503 / CATALOG_DRIFT.
• All 6 existing clinical-engine tests pass. Python compilation and git diff --check pass.

Commit carries DCO sign-off.