fix(a2a): declare a2a-sdk[http-server] so the a2a extra can serve

[arunpshankar]

Link to Issue or Description of Change

No existing issue.

Problem:

The a2a extra installs a2a-sdk with no extras. That is enough to
import the package but not to run the server: sse-starlette is declared
only under a2a-sdk's http-server, fastapi and all extras, and every
server dispatcher imports it at module level.

to_a2a() builds an A2AStarletteApplication, which routes through
a2a.server.routes.jsonrpc_dispatcher, so a clean install of
google-adk[a2a] fails at startup with:

ModuleNotFoundError: No module named 'sse_starlette'

This stays invisible during development because mcp requires
sse-starlette, so any environment that has ever installed the mcp extra
already satisfies it by accident. It surfaces on the first genuinely
clean install — typically a container — and the runtime then reports a
failed startup probe on the port rather than the missing module, which
sends you looking at networking instead of packaging.

Solution:

Declare a2a-sdk[http-server] in the a2a extra.

http-server rather than a direct sse-starlette pin because it states
the intent — ADK serves the A2A HTTP application — and leaves the
transitive set to a2a-sdk to decide. The extra exists at the pinned floor
(0.3.4) and carries sse-starlette through the current 1.1.2, so it holds
across the whole >=0.3.4,<2 range.

This is not the lazy-import case that some earlier dependency reports
turned out to be: the import lives inside a2a-sdk rather than in ADK, and
it is needed to serve, not merely to import.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally (unaffected by this change).

No unit test is added, and I want to be explicit about why rather than
add a token one: this is an install-time property, and CI installs the
extras it needs, so the failure is not observable from inside a test
session that can already import the module. Happy to add one if you would
like it — the natural home looks like the packaging-adjacent checks in
tests/unittests/test_import_loading.py.

That file's test_constructing_agent_defers_optional_mcp_server_stack
asserts sse_starlette is not imported when constructing an Agent.
This change does not affect it: installing a package does not import it.

Manual End-to-End (E2E) Tests:

From an empty virtualenv, installing exactly what the a2a extra
installs today:

$ pip install "a2a-sdk>=0.3.4,<2"
$ python -c "import a2a.server.routes.jsonrpc_dispatcher"
ModuleNotFoundError: No module named 'sse_starlette'
$ python -c "import a2a.compat.v0_3.jsonrpc_adapter"
ModuleNotFoundError: No module named 'sse_starlette'
$ python -c "import a2a.server.routes.rest_dispatcher"
ModuleNotFoundError: No module named 'sse_starlette'

With this change:

$ pip install "a2a-sdk[http-server]>=0.3.4,<2"
$ python -c "import a2a.server.routes.jsonrpc_dispatcher"   # ok
$ python -c "import a2a.compat.v0_3.jsonrpc_adapter"        # ok
$ python -c "import a2a.server.routes.rest_dispatcher"      # ok

pyproject-fmt==2.24.0, the version pinned in .pre-commit-config.yaml,
reports no change for pyproject.toml.

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.
• My changes generate no new warnings.