feat(fetch): make Body subclassable

[medz]

Type

Feature

Summary

• Replace the Body factory with a public generative constructor so downstream wrappers can call super(init).
• Move existing BodyInit normalization into an internal state builder so Body and subclasses share the same construction behavior.
• Preserve clone semantics for stream-backed bodies while allowing subclasses to override clone() and return their own type.
• Add public API coverage for a Body subclass that keeps wrapper metadata and clones stream bodies.

Impact

External packages can now extend Body directly without reimplementing body normalization or wrapping the Fetch body primitive.

Validation

• dart format --output=none --set-exit-if-changed .
• dart analyze
• dart test -p vm -p node -p chrome

Closes #52

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 81a65e0d-37fb-46d5-991f-01a1e78099b5

📥 Commits

Reviewing files that changed from the base of the PR and between 3495187 and 2a37e91.

📒 Files selected for processing (5)

• CHANGELOG.md
• lib/src/fetch/body.dart
• lib/src/fetch/request.native.dart
• lib/src/fetch/response.native.dart
• test/public_api_surface_test.dart

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/body-subclassable

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[chatgpt-codex-connector]

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