hang: non_exhaustive VideoConfig/AudioConfig with constructors

[kixelated]

Summary

• Mark hang::catalog::VideoConfig and AudioConfig as #[non_exhaustive] so additional optional fields can be added without bumping the major version.
• Add VideoConfig::new(codec) and AudioConfig::new(codec, sample_rate, channel_count) constructors that clear every optional field and default container via Container::default(). Callers assign whichever optional fields they need afterwards.
• Migrate every in-tree struct-literal site (15 files) across hang and moq-mux to the new constructors.

Why

Without #[non_exhaustive], every additive field on these public structs is a SemVer-breaking change for downstream Rust users that construct via struct literals. The previous attempt to add #[non_exhaustive] was rolled back because moq-mux (a separate crate) could no longer build via struct literals; the new(...) constructors are the unblocker.

This follows the pattern established in 9f780fa ("Tighten public APIs ahead of release: non_exhaustive + builders") for AuthToken and moq_msf::Track.

Notes for reviewers

• Required fields per constructor: VideoConfig::new only takes codec (everything else is optional or has Default); AudioConfig::new takes codec, sample_rate, and channel_count since the latter two have no sensible default and a wrong value silently distorts playback.
• Both constructors accept impl Into<{Video,Audio}Codec>, so callers can pass an H264 { ... } (or other variant payload) directly without .into().
• I touched a few sites beyond the original task list — additional struct-literal sites in rs/moq-mux/src/container/mkv/{import,export_test}.rs and several test cases in rs/moq-mux/src/catalog/hang/producer.rs — because they would otherwise fail to compile against the #[non_exhaustive] types.

Test plan

• just check (rustc, clippy -D warnings, fmt, cargo doc, cargo sort, biome, remark)
• just rs test — all suites pass, including the catalog roundtrip tests in hang and the hang/msf catalog tests in moq-mux

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: e2f47c0a-51fc-4da0-8ed7-0c59d4caafb3

📥 Commits

Reviewing files that changed from the base of the PR and between aacba0d and d030251.

📒 Files selected for processing (15)

• rs/hang/examples/video.rs
• rs/hang/src/catalog/audio/mod.rs
• rs/hang/src/catalog/root.rs
• rs/hang/src/catalog/video/mod.rs
• rs/moq-mux/src/catalog/hang/producer.rs
• rs/moq-mux/src/catalog/msf/consumer.rs
• rs/moq-mux/src/codec/aac/import.rs
• rs/moq-mux/src/codec/av1/import.rs
• rs/moq-mux/src/codec/h264/import.rs
• rs/moq-mux/src/codec/h265/import.rs
• rs/moq-mux/src/codec/opus/import.rs
• rs/moq-mux/src/container/fmp4/export_test.rs
• rs/moq-mux/src/container/fmp4/import.rs
• rs/moq-mux/src/container/mkv/export_test.rs
• rs/moq-mux/src/container/mkv/import.rs

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Walkthrough

This PR introduces public constructors for VideoConfig and AudioConfig and marks both structs #[non_exhaustive] to enforce constructor-based initialization. The VideoConfig::new() takes a codec and returns a config with all optional fields set to defaults, while AudioConfig::new() additionally accepts sample rate and channel count as required parameters. Every construction site across codec importers, container handlers, and tests is refactored from verbose struct literals to this new two-step pattern: initialize via the constructor, then set codec-specific or test-specific fields.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: marking VideoConfig and AudioConfig as non_exhaustive while adding constructors.
Description check	✅ Passed	The description clearly explains the purpose, implementation, and rationale for the changes with specific details about the constructors and migration.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch claude/funny-mccarthy-923790

✨ Simplify code

• Create PR with simplified code
• Commit simplified code in branch claude/funny-mccarthy-923790

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