fix: validate task ID length and message content - #1176

What changed

1. Reject over-long task IDs before persistence

Problem: No upper bound is enforced on taskId. A client can send a 1000+ character task ID in tasks/get, tasks/cancel, message/send, SubscribeToTask or any push-notification-config method and the ID is passed straight to the task store and persisted. There is also no check that a provided task ID is non-empty. Affected files: src/a2a/server/request_handlers/default_request_handler.py, src/a2a/server/request_handlers/default_request_handler_v2.py.

Fix (src/a2a/utils/task.py, src/a2a/server/request_handlers/default_request_handler.py, src/a2a/server/request_handlers/default_request_handler_v2.py):

• Added MAX_TASK_ID_LENGTH = 1000 and validate_task_id() in src/a2a/utils/task.py; it raises InvalidParamsError for empty task IDs or IDs longer than 1000 characters.
• Applied validate_task_id() at the entry of every task-ID-taking handler method in both V1 (LegacyRequestHandler) and V2 (DefaultRequestHandlerV2): on_get_task, on_cancel_task, on_subscribe_to_task, on_message_send/on_message_send_stream (via _setup_message_execution/_setup_active_task, only when a task ID is supplied) and the four push-notification-config methods. Validation happens before any store read/write.

2. Reject messages with empty content

Problem: MessageSend accepts messages with no parts or with parts that carry no content (no text, raw, url or non-null data). Such empty payloads are persisted into task history. The framework's schema validation already rejects an empty parts list with InvalidParamsError ("Validation failed"), but a part with no content (e.g. only media_type set) passes through unchecked.

Fix (src/a2a/utils/task.py, src/a2a/server/request_handlers/default_request_handler.py, src/a2a/server/request_handlers/default_request_handler_v2.py):

• Added validate_message_content() in src/a2a/utils/task.py, which raises InvalidParamsError if the message has no parts or any part is empty (no text/raw/url, and data is unset or null).
• Called it in _setup_message_execution (V1) and _setup_active_task (V2), which both on_message_send and on_message_send_stream route through, so the check applies to the streaming path too.

Testing

• ./.venv/Scripts/python -m pytest tests/utils/test_task.py tests/server/request_handlers/test_default_request_handler.py tests/server/request_handlers/test_default_request_handler_v2.py -q → 159 passed (includes 13 new regression tests: task-ID length limits and empty-content rejection at both the utility and handler level).
• ./.venv/Scripts/python -m ruff check on all modified files: clean (the two too-many-positional-arguments findings on the handler __init__ lines pre-date this change and exist on main).
• Behavior change: requests with over-long/empty task IDs and messages with empty content now return InvalidParamsError instead of being processed. This is the intended hardening; clients must send valid task IDs and non-empty message content.