Fix #5847: Replace shell binary check with Kotlin script and tests

[Sandesh282]

Fixes #5847

Explanation

Fixes #5847: The existing binary file check in pre-commit.sh uses file --mime | grep binary, which produces false positives on text files like .textproto, .json, .xml, and .md. This PR replaces that unreliable shell-based check with a robust Kotlin script (BinaryFileCheck.kt) that uses a two-layer detection approach:

1. Extension allowlist: Files with known text extensions (.kt, .java, .xml, .json, .textproto, .proto, .md, .sh, .bazel, .pro, etc.) pass extension validation. Files with unknown extensions are flagged as binary (fail-safe).
2. Content validation: Files with allowed extensions are scanned for binary content by checking that all characters are valid text characters (printable ASCII, Unicode whitespace, or any defined non-control Unicode character).

A filepath-based exemption list (binary_file_exemptions.textproto) is used to allow legitimate binary files (e.g., PNG images, GPG keys). Stale exemptions (paths that no longer exist) cause the check to fail, keeping the list up to date.

Changes to scripts/assets: Added binary_file_exemptions.textproto containing the 20 existing legitimate binary files in the repository (PNG images in drawable-nodpi/ and mipmap-*/, GPG key files in .gitsecret/, and the encrypted remote cache credentials config).

Essential Checklist

• The PR title starts with "Fix #bugnum: " (If this PR fixes part of an issue, prefix the title with "Fix part of #bugnum: ...".)
• The explanation section above starts with "Fixes #bugnum: " (If this PR fixes part of an issue, use instead: "Fixes part of #bugnum: ...".)
• Any changes to scripts/assets files have their rationale included in the PR explanation.
• The PR follows the style guide.
• The PR does not contain any unnecessary code changes from Android Studio (reference).
• The PR is made from a branch that's not called "develop" and is up-to-date with "develop".
• The PR is assigned to the appropriate reviewers (reference).

For UI-specific PRs only

Not applicable. This PR only modifies scripts, build configs, and CI workflows with no UI changes.

[github-actions]

Coverage Report

Results

Number of files assessed: 1
Overall Coverage: 90.38%
Coverage Analysis: PASS ✅

Passing coverage

Files with passing code coverage

File	Coverage	Lines Hit	Status	Min Required
BinaryFileCheck.kt scripts/src/java/org/oppia/android/scripts/binary/BinaryFileCheck.kt	90.38%	94 / 104	✅	70%

To learn more, visit the Oppia Android Code Coverage wiki page