Add encoding parameter to FileResource (#3580)

* feat: Add encoding parameter to FileResource

- Add optional encoding field (str | None, default None) to FileResource.
- Pass encoding through to read_text() for cross-platform text file reading.
- Preserve backward compatibility by defaulting to system encoding.

* test: Add tests for FileResource encoding parameter

- Test UTF-8 reading with explicit encoding for non-ASCII content.
- Test backward compatibility when no encoding is specified.
- Test that encoding is ignored for binary file reads.
- Test Latin-1 reading with matching encoding.

* docs: Document FileResource encoding parameter

- Add encoding="utf-8" to FileResource example in resource classes guide.
- Update FileResource description to mention encoding support.

* feat: Change FileResource encoding default from None to utf-8

- Default to utf-8 instead of system encoding to prevent cross-platform footgun.
- Update field description to reflect new default.
- Update test to verify default encoding is utf-8 with non-ASCII content.
- Remove redundant encoding="utf-8" from docs example since it is now the default.

docs/servers/resources.mdx

@@ -349,7 +349,7 @@ if data_dir_path.is_dir():
 
 -   `TextResource`: For simple string content.
 -   `BinaryResource`: For raw `bytes` content.
--   `FileResource`: Reads content from a local file path. Handles text/binary modes and lazy reading.
+-   `FileResource`: Reads content from a local file path. Handles text/binary modes, encoding, and lazy reading.
 -   `HttpResource`: Fetches content from an HTTP(S) URL (requires `httpx`).
 -   `DirectoryResource`: Lists files in a local directory (returns JSON).
 -   (`FunctionResource`: Internal class used by `@mcp.resource`).