feat: refactor getDiffs for testability, boost reviewer coverage

[brucearctor]

Adds DiffSource interface for testability. New tests for Run(), filterBySeverity, buildNumberedDiff.

Summary by CodeRabbit

• Bug Fixes
  • Improved review execution so diff retrieval behaves correctly in edge cases (such as empty diffs) and properly surfaces errors instead of continuing with invalid inputs.
  • Enhanced consistency of review output across terminal, JSON, and CI comment modes.
• Tests
  • Added extensive coverage for severity filtering, diff formatting/numbering, constructor wiring, and end-to-end review flow (including model and diff-source error handling).

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: 0e4e29a6-15d9-47ef-a38b-ce615424570e

📥 Commits

Reviewing files that changed from the base of the PR and between a8b55e8 and f96a446.

📒 Files selected for processing (2)

• internal/reviewer/reviewer.go
• internal/reviewer/reviewer_test.go

🚧 Files skipped from review as they are similar to previous changes (2)

• internal/reviewer/reviewer.go
• internal/reviewer/reviewer_test.go

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📝 Walkthrough

Walkthrough

Adds a DiffSource interface and diffSource field to Reviewer, a NewWithDiffSource constructor for dependency injection, and modifies Run to use the injected diffSource when present, falling back to existing getDiffs otherwise. Adds corresponding test coverage.

Changes

Diff Source Injection

Layer / File(s)	Summary
DiffSource interface, constructor, and Run wiring internal/reviewer/reviewer.go	Adds DiffSource interface with GetDiffs, a diffSource field on Reviewer, NewWithDiffSource, and conditional diff loading in Run.
Tests for DiffSource wiring and Run behavior internal/reviewer/reviewer_test.go	Adds mockDiffSource and tests for Run, filterBySeverity, buildNumberedDiff, constructor wiring, and output modes including CI notes and discussions.

Estimated code review effort: 2 (Simple) | ~15 minutes

Sequence Diagram(s)

sequenceDiagram
  participant Caller
  participant Reviewer
  participant DiffSource
  participant getDiffs

  Caller->>Reviewer: Run(ctx)
  alt diffSource is set
    Reviewer->>DiffSource: GetDiffs(ctx)
    DiffSource-->>Reviewer: diffs, title, description, error
  else diffSource is nil
    Reviewer->>getDiffs: getDiffs(ctx)
    getDiffs-->>Reviewer: diffs, title, description, error
  end

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🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main change: making getDiffs more testable and expanding reviewer test coverage.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/test-coverage-reviewer-80

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[coderabbitai]

🧹 Nitpick comments (1)

internal/reviewer/reviewer.go (1)

19-22: 📐 Maintainability & Code Quality | 🔵 Trivial | 💤 Low value

Optional: consider a richer return type instead of positional strings.

GetDiffs returns (diffs, title, desc, err) as four positional values, mirroring the existing getDiffs method. This is consistent with existing code, but a small struct (e.g. DiffContext{Diffs, Title, Desc}) would reduce the risk of accidentally swapping title/desc at call sites as more implementations are added.

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@internal/reviewer/reviewer.go` around lines 19 - 22, Replace the positional
string returns in DiffSource.GetDiffs with a small context struct so callers
don’t have to remember the order of title and desc. Update the DiffSource
interface and the existing getDiffs/GetDiffs implementation in reviewer.go to
return a single typed result like DiffContext containing Diffs, Title, and Desc,
and then adjust any call sites to use the named fields instead of unpacking
multiple values.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@internal/reviewer/reviewer.go`:
- Around line 19-22: Replace the positional string returns in
DiffSource.GetDiffs with a small context struct so callers don’t have to
remember the order of title and desc. Update the DiffSource interface and the
existing getDiffs/GetDiffs implementation in reviewer.go to return a single
typed result like DiffContext containing Diffs, Title, and Desc, and then adjust
any call sites to use the named fields instead of unpacking multiple values.

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: 44f30b73-2958-4822-8fd4-db1bcc9b6531

📥 Commits

Reviewing files that changed from the base of the PR and between 8d5d4e7 and a8b55e8.

📒 Files selected for processing (2)

• internal/reviewer/reviewer.go
• internal/reviewer/reviewer_test.go