fix(multi-tenant-saas): support MCP tool discovery

[dinakarjs]

The multi-tenant SaaS mock returned an unknown-tool error for tools/list, leaving the gateway unable to check upstream definitions. Route by method and expose all four PeopleGraph tools with catalog-matching descriptions and MCP inputSchema values. Existing calls remain unchanged; unsupported methods return -32601.

Follow-up to #134. Changes are limited to multi-tenant-saas/server/mock_mcp_server.py. Definitions are stored independently of the gateway catalog so upstream drift remains detectable.

Validation with published cmcp-runtime 0.7.0 in development mode:

• Live listing matches all four catalog definitions and handler names; all four call responses remain correct. Request IDs, unknown tool/method and wrong-path cases pass.
• metzler-eu: headcount and basic lookup allow; cross-region export and special-category lookup return POLICY_DENY.
• summit-us: first three calls allow; special-category lookup returns advisory_deny without blocking.
• Both tenants close with signed TRACE records, no drift/catalog exceptions, and no tools discovery incomplete warning.
• Altering people.headcount_analytics’s upstream description triggers UPSTREAM_CATALOG_DRIFT and HTTP 503 / CATALOG_DRIFT under both enforcing and advisory tenant configurations.
• All 5 existing people-directory tests pass. Python compilation and git diff --check pass.

Commit carries DCO sign-off.