fix(a2a): pass documentation_url to AgentCard in AgentCardBuilder

[anxkhn]

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• N/A (no existing issue; the change is described below following the issue template structure).

2. Or, if no issue exists, describe the change:

Problem:

AgentCardBuilder accepts a doc_url argument and stores it, but its build()
method passes that value to AgentCard using a keyword named doc_url:

# src/google/adk/a2a/utils/agent_card_builder.py
return AgentCard(
    ...
    doc_url=self._doc_url,
    ...
)

The a2a-sdk AgentCard model has no doc_url field. The documentation URL is
exposed as documentation_url (serialization alias documentationUrl). Because
AgentCard ignores unknown keyword arguments, the value passed as doc_url is
silently dropped, so AgentCardBuilder's doc_url argument has no effect and
generated agent cards never advertise their documentation URL.

Reproduction against the pinned a2a-sdk (>=0.3.4,<0.4; verified on 0.3.26):

from a2a.types import AgentCard

"doc_url" in AgentCard.model_fields            # False
"documentation_url" in AgentCard.model_fields  # True

common = dict(
    name="x", description="d", url="http://localhost/a2a", version="0.0.1",
    capabilities={}, skills=[],
    default_input_modes=["text/plain"], default_output_modes=["text/plain"],
)

AgentCard(doc_url="https://docs.example.com", **common).documentation_url
# -> None   (silently dropped)

AgentCard(documentation_url="https://docs.example.com", **common).documentation_url
# -> 'https://docs.example.com'

Solution:

Pass the stored value to the correct AgentCard keyword, documentation_url, so
the builder's doc_url argument is honored and the card advertises its docs URL:

-          doc_url=self._doc_url,
+          documentation_url=self._doc_url,

The public AgentCardBuilder(..., doc_url=...) argument is intentionally kept as
is, so this is a non-breaking, one-line behavior fix (only the internal keyword
passed to AgentCard changes).

The existing unit test test_build_with_custom_parameters documented the bug by
asserting result.documentation_url is None (with a comment noting the mismatch).
It is updated to assert the URL now round-trips onto AgentCard.documentation_url.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Updated test_build_with_custom_parameters in
tests/unittests/a2a/utils/test_agent_card_builder.py: it builds a card with
doc_url="https://docs.example.com" and now asserts
result.documentation_url == "https://docs.example.com".

This is a genuine regression test: reverting only the production line back to
doc_url=self._doc_url makes the assertion fail with
AssertionError: assert None == 'https://docs.example.com'; with the fix it passes.

pytest summary:

$ pytest tests/unittests/a2a/utils/test_agent_card_builder.py -q
77 passed, 17 warnings in 1.08s

$ pytest tests/unittests/a2a -q
362 passed

Manual End-to-End (E2E) Tests:

AgentCardBuilder is experimental and building a card does not require a running
agent, so the behavior is demonstrated directly against a2a-sdk (0.3.26):

BEFORE (doc_url=...)           -> documentation_url = None
AFTER  (documentation_url=...) -> documentation_url = 'https://docs.example.com'

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• 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.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

AgentCardBuilder is marked experimental (@a2a_experimental). The public
doc_url argument is deliberately left unchanged to avoid an API break; only the
internal keyword handed to AgentCard is corrected.