fix(a2a): advertise streaming on agent_registry agent cards

[lllakshit]

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Link to Issue or Description of Change

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

• Closes: build_agent_card(): agent_registry cards stay streaming:false after #6673, and the streaming= parameter becomes unreachable #6778
• Related: to_a2a() serves an agent card with capabilities.streaming=false while the mounted handler implements message/stream #6672, fix(a2a): default AgentCardBuilder capabilities to streaming=True #6673

Problem:
AgentRegistry.get_remote_a2a_agent() builds a card through _compat.build_agent_card() with neither capabilities nor streaming. The helper then falls through to default_capabilities where streaming defaults to False, so registry-constructed cards advertise streaming: false.

The issue also asked which public-API shape to use for the streaming= parameter. This PR takes the non-breaking option: keep streaming= as the no-capabilities convenience path only. It does not drop streaming= (that would remove public surface) and does not compose streaming= onto a passed capabilities object (that would change the meaning of an existing parameter; current tests already lock in "capabilities wins").

AgentCardBuilder still defaults AgentCapabilities() without streaming=True on GitHub main (#6673 closed without landing here). That is #6672 and is left out of this PR so this stays one concern.

Solution:
Pass streaming=True at the agent_registry call site, which is the convenience path build_agent_card already implements. Document that streaming= is ignored when capabilities is provided.

Testing Plan

Unit Tests:

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

Commands:

pytest tests/unittests/a2a/test_compat.py tests/unittests/integrations/agent_registry/test_agent_registry.py

Summary (Python 3.12.10, a2a-sdk 1.1.2, matching the issue repro):

71 passed, 24 skipped in 4.98s

Skipped tests are the existing 0.3-only @v03_only cases; this environment is a2a-sdk 1.x. New coverage that runs on 1.x:

• helper without capabilities still defaults streaming to False
• streaming=True is honoured when capabilities is omitted
• a passed capabilities object still wins (no composition)
• get_remote_a2a_agent() constructed cards advertise streaming is True

Manual End-to-End (E2E) Tests:

Reproduced the issue snippet against GitHub main vs this branch (a2a-sdk 1.1.2):

main agent_registry shape (omit streaming=):  False
this PR (streaming=True, no capabilities):    True
capabilities=AgentCapabilities(streaming=True), streaming=False: True

git show upstream/main:.../agent_registry.py still has no streaming= on that call. After this change the same call passes streaming=True.

No adk web UI change; this is card construction only.

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

I will complete the Google CLA if it is not already on file.

Assisted by an AI coding tool; I reviewed the diff, ran the tests above, and confirmed the issue still reproduces on main.

[google-cla]

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[lllakshit]

@googlebot rescan

I have already signed the Google Individual CLA (Lakshit Mathur / lllakshit / llakshitmathur239@gmail.com). Please re-check this pull request.

[lllakshit]

@DeanChensj Could you please review this PR and let me know if any changes are required? I’d appreciate your feedback so I can address any remaining concerns and move it toward merging into main.