refactor: Change Well-Known URI Magic Strings to a Constant defined in the 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 updates the A2A Python types in src/a2a/types.py to align with the latest specification. The core changes involve introducing a new class to define well-known URIs and enhancing existing documentation to reference these new standardized paths, thereby improving protocol consistency and clarity.

Highlights

• New Type Definition for Well-Known URIs: A new WellKnownUris class has been introduced in src/a2a/types.py. This class defines standard well-known URIs for the A2A protocol, specifically adding AGENT_CARD_WELL_KNOWN_URI as a Literal type with the value '/.well-known/agent.json'.
• AgentCard Documentation Enhancement: The docstring for the AgentCard class has been updated to include a recommendation that the AgentCard SHOULD be hosted at the well-known URI specified by the new WellKnownUris.AGENT_CARD_WELL_KNOWN_URI constant.

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Code Review

This pull request updates the A2A type definitions in src/a2a/types.py from the latest specification. It introduces a WellKnownUris class and updates the AgentCard docstring. I suggested using a simple class or Enum for WellKnownUris and using a direct reference instead of a string literal for WellKnownUris.AGENT_CARD_WELL_KNOWN_URI in the AgentCard docstring.