ci: re-enable proguard-tests:build in the CI unit path

[demolaf]

scripts/build.sh has had proguard-tests:build commented out since January 2025, so nothing in 10.x has ever been checked against the consumer ProGuard rules the libraries ship. This re-enables it for GA. It passes as-is: no rule or module changes were needed.

Changes

• Uncomment ./gradlew $GRADLE_ARGS proguard-tests:build in scripts/build.sh and drop the TODO(thatfiredev).
• Update CONTRIBUTING.md and four okf-bundle/ docs so the documented step list, the known-trap entry and the module table match the script again.

Why this is the only R8 coverage

Every library sets isMinifyEnabled = false and only declares consumerProguardFiles, so :proguard-tests is the one place R8 applies them. Verified sensitive: removing -dontwarn com.facebook.** from auth-proguard.pro fails minifyReleaseWithR8.

Cost

build depends on check, so the unit path now also runs :proguard-tests:lintRelease plus lint analysis for its library dependencies, duplicating work lint.yml already does. A fully uncached run of the step measured under a minute. Worth knowing: a lint finding there aborts build.sh before checkstyle and the unit tests.

---

Maintainer note: Fixes internal CPRN-491

[gemini-code-assist]

Code Review

This pull request enables the R8/ProGuard packaging gate (proguard-tests:build) in the CI build script (scripts/build.sh) and updates the associated documentation. The reviewer suggests optimizing this step by running :proguard-tests:assembleRelease instead of the full build task, which avoids executing redundant Android Lint checks and saves CI build time. The reviewer also provides corresponding documentation updates to align with this optimization.