feat: Add constants for Well-Known URIs

[holtskinner]

Prevents usage of "Magic Strings"

[gemini-code-assist]

Summary of Changes

Hello @holtskinner, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request is a refactoring effort aimed at improving the codebase's readability and maintainability. My primary goal was to eliminate the use of hardcoded 'magic strings' for well-known URIs by centralizing their definitions into a new constants module. All relevant client and server components have been updated to utilize these new constants, making the system more robust and easier to manage.

Highlights

• Code Refactoring: Centralized URI Constants: I've introduced a new file, src/a2a/utils/constants.py, to centralize the definitions of well-known URIs. This includes AGENT_CARD_WELL_KNOWN_PATH, EXTENDED_AGENT_CARD_PATH, and DEFAULT_RPC_URL.
• Improved Maintainability: Eliminating Magic Strings: I've replaced hardcoded 'magic string' URIs with the newly defined constants across various client and server modules. This enhances readability, reduces the chance of errors from typos, and simplifies future updates to these paths.
• Minor Code Enhancements: I've also added -> None return type hints to several __init__ methods for better type clarity and applied minor formatting adjustments (e.g., wrapping method calls in parentheses) for consistency.

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[gemini-code-assist]

Code Review

This pull request refactors hardcoded URI paths into constants, enhancing code maintainability and readability.