[MM-66932] Add Rust build pipeline and OpenMLS library

[cpoile]

Summary

Add CI and developer setup for E2EE Rust module

• I've tried my best to reduce the added size of the rust code, but it's still a bit. I'll keep looking at ways to reduce it.
• I've also tried to keep the added CI work to a minimum -- we're caching the rust build artifacts, so we shouldn't see too much added time here for regular PRS that don't change e2ee code.

E2EE Module:

• Add @mattermost/e2ee library with Rust core in libraries/@mattermost/e2ee/
• Just for testing, I made a test component in the channel info menu that called greet from the Rust code through the TS wrapper, and it all works. (to be clear: I did not include that component in this PR)
• Bindings are auto generated using uniffi-bindgen-react-native
• TypeScript wrapper layer for React Native integration
• Configure iOS Podspec and Android Gradle build
• Added old architecture overrides for RN 0.76 compatibility (New Architecture disabled) -- it shouldn't be too much work to get it working when we move to new arch.

Developer Setup:

• Add scripts/setup-rust.sh that auto-installs Rust, required targets, and cargo-ndk if missing
• We call the setup script from postinstall.sh so npm i works out of the box for devs

Build System:

• Add scripts/build.sh to compile Rust for iOS targets (aarch64, aarch64-sim, x86_64) and Android targets (aarch64, armv7, x86_64, i686)
• Add npm run e2ee:build and related scripts to root package.json
• Auto-build E2EE module during npm install via postinstall hook
• Add scripts/clean.sh updates to clean Rust build artifacts

CI Changes:

• Add test-rust job to ci.yml that runs Rust unit tests in parallel with existing test job
• The rest of the tests are run in the build app steps
• Add prepare-rust-toolchain action to set up Rust, run tests, build E2EE module, and verify builds
• rust-toolchain.toml is at the repo root so CI actions can auto-detect it

E2EE Testing:

• Add npm run e2ee:test scripts for Rust unit tests, iOS, and Android verification
• Add verify-build.sh to validate built artifacts exist and have correct structure
• Add iOS test that compiles Swift against xcframework and verifies FFI checksums on simulator (we can remove this eventually -- it's just to make sure things are working)
• Add Android test that verifies static library symbols are correctly exported (can't easily use emulator -- we can remove this eventually too)

Ticket Link

• https://mattermost.atlassian.net/browse/MM-66932

Checklist

• Added or updated unit tests (required for all new features)
• Has UI changes
• Includes text changes and localization file updates
• Have tested against the 5 core themes to ensure consistency between them.
• Have run E2E tests by adding label E2E iOS tests for PR.

Device Information

This PR was tested on:

Screenshots

Release Note

Added rust to mobile builds, CI pipeline, basic tests

[github-actions]

Coverage Comparison Report

Generated on December 15, 2025 at 22:36:06 UTC

+-----------------+------------+------------+-----------+
| Metric          | Main       | This PR    | Diff      |
+-----------------+------------+------------+-----------+
| Lines           |     86.18% |     86.18% |     0.00% |
| Statements      |     86.06% |     86.06% |     0.00% |
| Branches        |     72.61% |     72.61% |     0.00% |
| Functions       |     85.40% |     85.40% |     0.00% |
+-----------------+------------+------------+-----------+
| Total           |     82.56% |     82.56% |     0.00% |
+-----------------+------------+------------+-----------+

[github-actions]

Android E2E Test Report: a7e9735 | "84.18"% (282/335) | full report

Tests	Passed ✅	Failed ❌	Skipped ⏭️	Errors ⚠️
335	282	52	1	0

[github-actions]

Android E2E Test Report: 4c59f0e | "84.48"% (283/335) | full report

Tests	Passed ✅	Failed ❌	Skipped ⏭️	Errors ⚠️
335	283	51	1	0