feat: add --fail-on-unfixable flag for single-pass fix and validation

[omar-y-abdi]

Fixes #6450

Problem: swiftlint --fix always returns exit code 0, even when unfixable violations
remain. Users in CI/pre-commit hooks must run SwiftLint twice: swiftlint --fix && swiftlint --strict,
doubling execution time.

Solution: Add --fail-on-unfixable flag that performs autocorrection as usual, then
re-lints to detect remaining violations. If serious violations remain, exits with code 2.
Reuses existing lintOrAnalyze() path, so --strict/--lenient work naturally with the
re-lint pass.

Why: The two-pass approach (fix then lint) reuses existing infrastructure with minimal code
changes. Configuration parsing is cached (O(1) on second pass), and ignoreCache: true
ensures stale cache entries from modified files are not used. This is architecturally the
simplest solution that correctly handles all edge cases.

Test:

• swift build passes
• LintOrAnalyzeOptionsTests passes (0 failures)
• Manual e2e verification:
  • Mixed violations (fixable + unfixable) + --fail-on-unfixable → exit 2 → works
  • Same without flag → exit 0 → works
  • Only fixable violations + --fail-on-unfixable → exit 0 → works
  • --fail-on-unfixable without --fix → prints warning → works

Closes #6450

[SwiftLintBot]

	1 Warning
⚠️	This PR may need tests.

	19 Messages
📖	Building this branch resulted in a binary size of 27358.1 KiB vs 27357.74 KiB when built on main (0% larger).
📖	Linting Aerial with this PR took 0.81 s vs 0.81 s on main (0% slower).
📖	Linting Alamofire with this PR took 1.06 s vs 1.06 s on main (0% slower).
📖	Linting Brave with this PR took 7.21 s vs 7.22 s on main (0% faster).
📖	Linting DuckDuckGo with this PR took 26.66 s vs 26.66 s on main (0% slower).
📖	Linting Firefox with this PR took 11.95 s vs 12.01 s on main (0% faster).
📖	Linting Kickstarter with this PR took 8.24 s vs 8.28 s on main (0% faster).
📖	Linting Moya with this PR took 0.47 s vs 0.47 s on main (0% slower).
📖	Linting NetNewsWire with this PR took 2.54 s vs 2.53 s on main (0% slower).
📖	Linting Nimble with this PR took 0.68 s vs 0.72 s on main (5% faster).
📖	Linting PocketCasts with this PR took 7.68 s vs 7.71 s on main (0% faster).
📖	Linting Quick with this PR took 0.48 s vs 0.47 s on main (2% slower).
📖	Linting Realm with this PR took 2.98 s vs 3.02 s on main (1% faster).
📖	Linting Sourcery with this PR took 1.88 s vs 1.87 s on main (0% slower).
📖	Linting Swift with this PR took 4.68 s vs 4.68 s on main (0% slower).
📖	Linting SwiftLintPerformanceTests with this PR took 0.36 s vs 0.35 s on main (2% slower).
📖	Linting VLC with this PR took 1.21 s vs 1.19 s on main (1% slower).
📖	Linting Wire with this PR took 18.68 s vs 18.78 s on main (0% faster).
📖	Linting WordPress with this PR took 12.67 s vs 12.72 s on main (0% faster).

Generated by 🚫 Danger