Android: apply bsdiff patches during package install

[ofalvai]

Why

Building on top of #40, this PR integrates applyPatch() into the diff handling codepath.

What

The main change is in CodePushUpdateManager: it learns to handle the new manifest file version (which defines patch files, checksums and the patch file format), and calls BinaryDiffPatcher.

BinaryDiffPatcher does the main business logic, it walks over the patch files, applies them, and validates checksums.

I also made a series of smaller refactors to existing Java code in order to avoid code duplication or bloating existing Java classes with new code:

• Extracted SHA-256 checksumming to its own standalone function (BinaryDiffPatches also needs this)
• Extracted diff manifest JSON parsing to a standalone file. Manifest file parsing was ad-hoc and it was at multiple places for a single JSON field.

Out of scope

Signaling binary diff support to the server when checking for updates: the server should know what kind of delta update the client can handle (existing file-by-file diffing or binary diffing) because the manifest format is different (see below). We'll need to work out the details of this, so it's going to be a future PR.

User-facing on-off switch for the new behavior: see #43

Decisions

Manifest file version compatibility: the new manifest file format contains an explicit version field, and binary diff updates are going to set version=2. When the version field is missing, we interpret it as version=1 (file-by-file diffing only, deleted/added/modified files). We also check for version >2 and throw a hard error. The server is not supposed to send an update format that the client doesn't know how to handle, I think it's best to abort the update if this happens.

Tests: thanks to #38, new code in this PR can be unit tested instead of writing elaborate and slow E2E tests.

[Copilot]

Pull request overview

Adds Android support for applying BSDIFF patches while installing version 2 delta updates.

Changes:

• Parses versioned diff manifests.
• Applies patches with path and checksum validation.
• Adds unit tests and fixtures.

Reviewed changes

Copilot reviewed 13 out of 14 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
android/app/build.gradle	Adds JSON test dependency.
CodePushConstants.java	Defines the patch directory name.
CodePushUpdateManager.java	Integrates v2 patch installation.
CodePushUpdateUtils.java	Reuses manifest and hashing utilities.
diffpatch/BinaryDiffPatcher.kt	Implements validated patch application.
diffpatch/DiffManifest.kt	Models and parses manifests.
diffpatch/DiffPatch.kt	Adds the patch-applier abstraction.
diffpatch/Sha256.kt	Provides SHA-256 helpers.
BinaryDiffPatcherTest.kt	Tests patching behavior and validation.
DiffManifestTest.kt	Tests manifest parsing.
Sha256Test.kt	Tests checksum generation.
binarydiff/basic/old.dat	Supplies the base fixture.
binarydiff/basic/new.dat	Supplies the expected output fixture.

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