Report missing essential modules to the frontend

[summeroff]

Draft / do not merge until #1733 lands in staging.
Built on top of #1733 (ndi-error-handling); base will retarget to staging once #1733 merges. Depends on libobs 31.1.2ndi1 (obs-studio #741).

Description

Adds a positive capability check after module load that verifies the OBS plugins we depend on are actually registered, and surfaces any gaps through the structured moduleLoadFailures list introduced in #1733 (read by OBS_API_getModuleLoadFailures).

Motivation

#1733 / obs-studio #741 expose structured module load failures, but libobs only records a failure for MODULE_ERROR, MODULE_INCOMPATIBLE_VER, and modules that load-but-decline. The two states that actually break us are skipped silently:

• MODULE_FILE_NOT_FOUND — the plugin DLL is missing on disk (e.g. obs-ffmpeg.dll)
• MODULE_MISSING_EXPORTS — the DLL fails to expose its OBS exports

In both cases load_all_callback just return;s, so the module never appears in mfi.failures. A frontend reading only the failure list therefore sees nothing — even though a core encoder is gone.

This is the condition behind a class of startup crashes: with obs-ffmpeg.dll absent, the ffmpeg_aac encoder isn't registered, and recording-audio-encoder setup dereferenced the null codec (strlen(NULL) AV in OBS_API_initAPI). The crash itself is fixed separately in #1736; this PR makes the underlying missing-plugin condition visible to the frontend.

What it does

After obs_post_load_modules(), reportMissingEssentialModules() checks registration of:

Capability	Check	Owning plugin
ffmpeg_aac	obs_get_encoder_codec	obs-ffmpeg
obs_x264	obs_get_encoder_codec	obs-x264
rtmp_output	obs_get_output_flags	obs-outputs
rtmp_common	obs_enum_service_types	rtmp-services

Any missing one is logged and appended to moduleLoadFailures with code ESSENTIAL_MODULE_MISSING (deduped against entries libobs already reported). Init is not aborted.

How Has This Been Tested?

• obs-studio-server-lib (Release, VS 2022) compiles cleanly against the Update libobs to v25.20.20 #741 obs_module_failure_info (verified locally by adding the struct to the prebuilt header; the build tree's cached libobs is 31.1.2sl6, so CI on the 31.1.2ndi1 package is authoritative).
• clang-format 18.1.3 --dry-run -Werror on the added lines: clean.

Types of changes

• New feature (non-breaking change which adds functionality)

🤖 Generated with Claude Code

[summeroff]

Superseded by streamlabs/obs-studio#743. Detecting missing/skipped plugins belongs in libobs (where the load decision is made) rather than reconstructing it in OSN with a positive capability check. #743 records the silent skip paths (MODULE_NOT_OBS_PLUGIN / MODULE_MISSING_EXPORTS / MODULE_FILE_NOT_FOUND) as structured failures, which then flow to the frontend through the existing OBS_API_getModuleLoadFailures (#1733) with no OSN-side change. The startup crash guard remains in #1736.