docs(rest/python): align discovery response example

[FanouZeng-TT]

Description

The Discovery section of rest/python/server/README.md labels its JSON block
as the response returned by /.well-known/ucp, but the example still uses the
old profile layout:

• services.dev.ucp.shopping is an object instead of a transport-entry array;
• capabilities is an array instead of a capability-name map;
• payment handlers are outside ucp under payment.handlers.

The route now serves routes/discovery_profile.json, with dynamic
signing_keys and ucp.keys added at request time. Its response uses the
current ucp.services, ucp.capabilities, and ucp.payment_handlers shapes.

Fix: update the documented response to match the live endpoint, using
explicit placeholders for runtime-generated shop and signing-key values.

Category (Required)

• Core Protocol: Changes to the UCP core protocol. (Requires Technical Council approval)
• Governance/Contributing: Changes to governance or contributing processes. (Requires Governance Council approval)
• Capability: Changes to UCP capabilities. (Requires Maintainer approval)
• Documentation: Documentation-only changes. (Requires Maintainer approval)
• Infrastructure: CI, build, or repository infrastructure changes. (Requires DevOps Maintainer approval)
• Maintenance: Dependency or maintenance changes. (Requires DevOps Maintainer approval)
• SDK: Language-specific SDK updates and releases. (Requires DevOps Maintainer approval)
• Samples / Conformance: Maintaining samples and the conformance suite. (Requires Maintainer approval)
• UCP Schema: Changes to the ucp-schema tool. (Requires Maintainer approval)
• Community Health (.github): Community health file changes. (Requires DevOps Maintainer approval)

Related Issues

N/A

Checklist

• I have followed the Contributing Guide.
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• (For Core/Capability) I have included/updated the relevant JSON schemas.
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk.

Screenshots / Logs (if applicable)

• GET /.well-known/ucp through the FastAPI test client returned 200 with
  public, max-age=3600; verified its services, capabilities, payment handlers,
  and signing-key container shapes.
• Parsed the updated README JSON and asserted the same stable shape, local
  endpoint, and runtime-generated-value placeholders.
• Full pre-commit and git diff --check pass.

[niranjanmanjunath]

(nit) While not strictly blocking, adding "use": "sig" and "alg": "ES256" to the README example would make it an exact 1:1 match with the server's actual HTTP output.