fix: remove MIME type validation from MCPServer Resource

[maxisbey]

Removes the MIME type validation regex from MCPServer.Resource.mime_type.

Closes #1756. Supersedes #1819 and #2067.

Motivation and Context

The pattern= constraint on Resource.mime_type was the only MIME type validation anywhere in the MCP ecosystem, and it rejected valid RFC 2045 MIME types — most notably quoted parameter values like text/plain; charset="utf-8".

#1755 partially fixed this by extending the regex to allow unquoted parameters, but the fundamental problem is that no other SDK validates this field at all, and the spec doesn't ask them to:

Where	Definition	Validation
Spec schema — Resource.mimeType	mimeType?: string	None
TypeScript SDK	z.optional(z.string())	None
Go SDK	MIMEType string	None
C# SDK	string? MimeType	None
Python SDK — protocol types	mime_type: str | None = None	None
Python SDK — ResourceTemplate	Field(default="text/plain")	None
Python SDK — Resource (before this PR)	Field(pattern=r"^[a-zA-Z0-9]+/...")	❌ Regex

The inconsistency with ResourceTemplate in the same module is the clearest signal — if templated resources don't need validation, concrete ones don't either.

How Has This Been Tested?

• Extended existing test_resource_mime_type with a quoted-parameter case that the old regex rejected
• All resource tests pass (61 tests across tests/server/mcpserver/resources/ + tests/issues/test_1754_* + tests/issues/test_152_*)
• 100% branch coverage on base.py

Breaking Changes

None. Strictly more permissive.

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

Two prior PRs attempted this:

• Added custom validator for Resource mime type which is less retrictiv… #1819 added an email.message-based RFC 2045 validator (40+ lines) — right problem, wrong direction (more validation, not less); also predates the fastmcp → mcpserver rename
• fix: relax MIME type validation to accept all RFC 2045 types #2067 added a minimal "/" in value check — reasonable, but still diverges from ResourceTemplate and the other SDKs

This PR just deletes the pattern: +1/-5 source diff.

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