feat: allow configuring SSE shutdown grace periods

[ftnext]

• 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)

---

Description

Adds an optional shutdown_grace_period parameter to the JSON-RPC and REST route factories and forwards it to every EventSourceResponse, including the v0.3 compatibility routes.

The default remains 0 for backward compatibility with sse-starlette. The minimum supported sse-starlette version is updated to 3.3.0, where cooperative shutdown support was introduced.

Tests cover:

• Default and custom values for both route factories
• JSON-RPC and REST streaming responses
• JSON-RPC and REST v0.3 compatibility routes
• Empty REST streams

Verification

• ./scripts/lint.sh
• uv run pytest
• uv run pytest --cov=src --cov-report=term-missing
• 93% total coverage

Fixes #1221 🦕

[github-actions]

🧪 Code Coverage (vs main)

⬇️ Download Full Report

	Base	PR	Delta
src/a2a/compat/v0_3/jsonrpc_adapter.py	66.36%	66.67%	🟢 +0.30%
src/a2a/compat/v0_3/rest_adapter.py	76.47%	77.14%	🟢 +0.67%
src/a2a/server/routes/jsonrpc_dispatcher.py	85.44%	85.55%	🟢 +0.11%
src/a2a/server/routes/rest_dispatcher.py	94.25%	94.86%	🟢 +0.60%
src/a2a/utils/version_validator.py	92.86%	94.29%	🟢 +1.43%
Total	92.97%	92.99%	🟢 +0.03%

Generated by coverage-comment.yml