requesthandler: Include the ImageFormat helper in the sources handler

[rdoupe]

Description

Fixes the build of the plugin in a full OBS build (frontend + obs-websocket), broken since #6.

GetSourceScreenshot calls Utils::ImageFormat::MimeType() since "utils: Move the screenshot MIME type into a helper" (#6), but src/requesthandler/RequestHandler_Sources.cpp never includes ../utils/ImageFormat.h, and nothing it includes pulls it in (RequestHandler.h includes ../utils/Obs.h, not Utils.h). A full build fails with:

RequestHandler_Sources.cpp:231: error: 'Utils::ImageFormat' has not been declared

This adds the one missing include.

Motivation and Context

My mistake in #6: its test (test_ws_image_mime in obs-rust/obs-studio#652) compiles the helper on its own, and the obs-studio PR builds skip the full app, so nothing compiled this file before #6 and #652 merged. obs-studio master now pins 3e46553 (#652), so the next full build there (the daily release gate) would fail until the pin moves past this fix.

How Has This Been Tested?

• A full OBS build (frontend + obs-websocket) at this fork's master fails with the error above; with this commit it builds, with both libobs lanes (ENABLE_RUST_LIBOBS OFF and ON).
• The tests of utils: Count rescaled encoders in GetStats output frames #4, utils: Start the scene counter at zero #5, utils: Label screenshots with the real MIME type #6 and eventhandler: Send SourceFilterSettingsChanged for filter changes #7 pass on top of it in that build (obs-studio tests test_ws_image_mime, test_ws_filter_settings_event, test_ws_scene_count, and the Tier 4 GetStats phase).

Types of changes

• Bug fix (non-breaking change which fixes an issue)

🤖 Generated with Claude Code

[rdoupe]

Friendly nudge on this one: obs-studio master now pins this fork at a commit where RequestHandler_Sources.cpp uses Utils::ImageFormat without including ../utils/ImageFormat.h. PR CI doesn't compile that file, but the full OBS build does, so the next daily release build will fail until this include lands. It's a one-line change. Once it's merged I'll open the obs-studio PR that moves the pin forward.

[rdoupe]

Closing as redundant: #10 landed the same include and obs-studio master now pins 547f5ea, which builds. Thanks.