fix: URL-decode parameters extracted from resource templates

[felixweinberger]

Summary

URL-decode parameters extracted from resource template URIs so that percent-encoded characters (like %20 for space, %C3%A9 for é) are properly decoded before being passed to resource handlers.

Motivation and Context

When a client requests a resource using a URI like search://hello%20world, the {query} parameter was being passed to the handler as the literal string "hello%20world" instead of "hello world". This broke handlers that expected decoded strings.

Fixes #973

How Has This Been Tested?

Added tests/issues/test_973_url_decoding.py with 4 test cases:

• Space decoding (%20 → )
• Accented characters (%C3%A9 → é)
• Complex French phrase from the original issue
• Verification that + remains as + (correct URI behavior vs form encoding)

Breaking Changes

None. This is a bug fix that makes resource templates work as users would expect.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

Uses urllib.parse.unquote which handles UTF-8 encoded characters correctly. The + sign is intentionally NOT converted to space because that behavior is specific to application/x-www-form-urlencoded (HTML forms), not URI encoding.