feat: Update A2A types from specification 🤖

[a2a-bot]

This PR updates src/a2a/types.py based on the latest specification/json/a2a.json from a2aproject/A2A.

[gemini-code-assist]

Summary of Changes

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This pull request is an automated update to synchronize the Python A2A type definitions in src/a2a/types.py with the latest external A2A specification. The changes primarily involve a minor functional adjustment to a field's optionality and numerous formatting improvements to enhance code readability.

Highlights

• Type Definition Update: The acceptedOutputModes field within the MessageSendConfiguration class has been updated to be optional (list[str] | None) and now defaults to None. This change reflects an update in the underlying A2A specification.
• Code Formatting and Readability: Several type definitions, particularly those using RootModel and Field declarations, have been reformatted. This involves condensing multi-line arguments into single lines to improve code conciseness and readability without altering functionality.

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[gemini-code-assist]

Code Review

This pull request updates the A2A type definitions from the latest specification. The changes are mostly auto-generated and include one functional update to make a field optional, and several formatting changes that consolidate multi-line definitions into single lines.

My review focuses on the formatting changes, as several of them result in lines that exceed the standard PEP 8 line length recommendation, which can reduce readability. I've left suggestions to reformat these lines and recommended adding an automatic code formatter to the generation script to prevent this in the future. The functional change appears correct and consistent with the rest of the data model.