fix: relax MIME type validation to accept all RFC 2045 types

[BabyChrist666]

Summary

Closes #1756

• Replaced the restrictive regex pattern on Resource.mime_type with a lightweight field_validator that only checks for the presence of / (type/subtype structure)
• Aligns with the MCP spec which defines mimeType as an optional string with no format constraints, and matches the TypeScript SDK behavior (no validation)
• Fixes rejection of valid RFC 2045 MIME types including quoted parameter values (e.g. text/plain; charset="utf-8"), wildcard types (*/*), and complex boundary parameters

What changed

src/mcp/server/mcpserver/resources/base.py

• Removed pattern=r"^[a-zA-Z0-9]+/[a-zA-Z0-9\-+.]+(;\s*[a-zA-Z0-9\-_.]+=[a-zA-Z0-9\-_.]+)*$" from the mime_type Field
• Added validate_mime_type field_validator that only rejects values without a / separator

Why this approach

The previous regex was too restrictive for RFC 2045 compliance. Rather than trying to write a more complex regex that covers all edge cases (quoted strings, special characters, wildcards, etc.), this PR follows the MCP spec's intent: mimeType is a plain string, and validation should only catch obvious mistakes (like passing "plaintext" instead of "text/plain"). This is consistent with:

• The TypeScript SDK (no MIME validation)
• ResourceTemplate in this SDK (already had no pattern constraint)
• Protocol-level types in _types.py (no validation)

Test plan

• Added tests/issues/test_1756_mime_type_relaxed.py with 12 test cases covering:
  • Basic MIME types, quoted parameter values, vendor types, wildcards, complex parameters
  • Invalid MIME types (no /, empty string)
  • Default MIME type preservation
• Added test_resource_mime_type_validation to existing test_resources.py
• All existing tests pass (including test_1754_mime_type_parameters.py)
• 100% code coverage on base.py
• ruff + pyright clean

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