fix(quality): preserve capped reader failures to prevent uncapped reads in diff scans

[alxxjohn]

Motivation

• Prevent diff-mode maintainability checks from bypassing the configured size-capped file reader and performing an uncapped os.ReadFile when the reader returns an error.

Description

• Change readCurrentTargetFile to honor failures from the configured env.ReadTargetFile by returning the error result rather than falling back to an unconditional os.ReadFile when the callback exists.
• Keep the direct filesystem os.ReadFile fallback only when no env.ReadTargetFile callback is configured, preserving prior fallback behavior for unconfigured contexts.
• Add a regression test internal/codeguard/checks/quality/quality_precision_delta_test.go that verifies a reader error (e.g. scan-limit error) is not bypassed by readCurrentTargetFile.

Testing

• Ran go test ./internal/codeguard/checks/quality and the package tests passed (ok).
• Ran the maintainability diff tests with go test ./tests/checks -run 'TestMaintainability(PublicSurface|Dependency)GrowthWarnsInDiffScan' -count=1 and they passed (ok).
• Ran the combined checks used in CI (go test ./internal/codeguard/checks/quality ./tests/checks) and the tests completed successfully.

---

Codex Task