docs(samples): add Agent Card signing sample

[orangecola]

There was no runnable example for a2a.utils.signing, so this adds one
covering both halves of Agent Card signing.

samples/agent_card_signing.py:

• Server: generates an ES256 key, signs the Agent Card with
  create_agent_card_signer, serves it via
  create_agent_card_routes(card_modifier=...), and publishes the public key
  as a JWKS document at /.well-known/jwks.json — the URL the signature's
  jku points to.
• Client: verifies the card with create_signature_verifier passed to
  A2ACardResolver.get_agent_card(signature_verifier=...), resolving keys by
  kid from an allowlist of pinned JWKS URLs, with algorithms pinned to
  ES256.
• Modes: demo (default, runs both in-process), serve, verify --url.

The default demo mode verifies the genuine card and then shows the three
rejections a verifier exists for:

1. The card served by the agent
   verified card for agent: Signed Card Agent
   protected header: {'alg': 'ES256', 'jku': '.../.well-known/jwks.json', 'kid': 'sample-card-key-1', 'typ': 'JOSE'}

2. A card whose transport URL was rewritten in transit
   rejected as expected: No valid signature found

3. A card with its signature stripped off
   rejected as expected: AgentCard has no signatures to verify.

4. The genuine card, checked by a client that pins other keys
   rejected as expected: No valid signature found

Two non-obvious details are called out in comments, since they bite anyone
copying the pattern:

• The verifier is a synchronous callable, so fetching the JWKS inside it
  with a sync HTTP client blocks the event loop (it deadlocks against an
  in-process server and reports No valid signature found). The key provider
  does pure in-memory lookups over JWKS prefetched asynchronously.
• create_agent_card_signer mutates the card it signs, so a per-request
  card_modifier would accumulate signatures. The helper deep-copies first and
  the card is signed once at startup.

The key provider also raises PyJWKError rather than a plain exception, so the
verifier treats one unusable signature as unverifiable and moves to the next
instead of aborting a card that also carries a good signature.

Also adds tests/integration/test_agent_card_signing_smoke.py (runs the demo
on a free port, asserts one acceptance and three rejections) and a
samples/README.md section.

No library code changed — samples, tests and docs only.

Testing

• uv run pytest tests/utils/test_signing.py tests/integration/test_agent_card_signing_smoke.py tests/client/test_card_resolver.py → 55 passed
• uv run ruff format --check and uv run ruff check → clean
• uv run ty check → same 4 pre-existing diagnostics as main (samples are outside [tool.ty.src])

Checklist

• Follow the CONTRIBUTING Guide.
• Make your Pull Request title in the https://www.conventionalcommits.org/ specification.
• Ensure the tests and linter pass (Run bash scripts/format.sh from the repository root to format)
• Appropriate docs were updated (if necessary)

Fixes #1197 🦕

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[mykytanetipa]

Presubmits need to be green before review can be granted.

Only failure is Check Spelling: it flags SECP at samples/agent_card_signing.py. Since check-spelling splits on digits, add a bare SECP to .github/actions/spelling/allow.txt (the existing SECP256R1 won't match).