fix(financial-services): support MCP tool discovery

[dinakarjs]

The financial-services mock treated tools/list as tools/call and returned an unknown-tool error, so the gateway could not compare its upstream definitions with the approved catalog. Route requests by method and list all six tools with the exact catalog descriptions and MCP inputSchema values. Existing call handlers remain unchanged; unsupported methods return -32601.

Follow-up to #134 and #136, limited to financial-services/server/mock_mcp_server.py. The definitions are stored in the mock independently of the gateway catalog, so an upstream definition change remains detectable.

Validation:

• Live HTTP tools/list matches all six catalog definitions and the handler names; request IDs are preserved.
• All six tools/call responses match the existing handlers; unknown tool/method and wrong-path cases pass.
• Published cmcp-runtime 0.7.0 in development mode: clean scenario allows all six calls; large-exposure and sanctions-hit allow the first five and deny the final write with POLICY_DENY. Each closed session has a signed TRACE record, no drift and no catalog exceptions. No tools discovery incomplete warning.
• Negative drift control: changed finance.document_reader’s upstream description without changing the catalog. The runtime logs UPSTREAM_CATALOG_DRIFT and blocks the call with HTTP 503 / CATALOG_DRIFT.
• All 9 existing financial credit-engine tests pass. Python compilation and git diff --check pass.

Commit carries DCO sign-off.