fix(server): classify structured file MIME types as text

[manaskarra]

Summary

• Treat structured text MIME types such as application/json, application/*+json, XML, YAML, JavaScript, and SVG as text for FileResource.
• Preserve binary behavior for application/octet-stream and explicit is_binary=True overrides.
• Add regression coverage for common text-like non-text/* MIME types.

Motivation and Context

FileResource currently marks every non-text/* MIME type as binary. That makes common textual resources like JSON files return bytes, which are then exposed as blob resource contents instead of text resource contents.

How Has This Been Tested?

• uv run --frozen pytest tests/server/mcpserver/resources/test_file_resources.py
• uv run --frozen ruff check src/mcp/server/mcpserver/resources/types.py tests/server/mcpserver/resources/test_file_resources.py
• git diff --check