Cap rule-stats history reads to prevent memory exhaustion

[alxxjohn]

Motivation

• The rule-stats history loader previously used os.ReadFile with no size cap, allowing a repository-controlled history file to cause the process to buffer an arbitrarily large file and exhaust CI memory.
• The fix aims to prevent OOM by reusing the existing size-capped cache reader so oversized or malicious history files are treated as cache-misses while preserving existing missing/malformed/version-mismatch semantics.

Description

• Replace the uncapped os.ReadFile/json.Unmarshal path in LoadRuleStatsHistory with the shared size-capped loader cachefile.Load, and add the cachefile import.
• Keep the same behavior for blank paths, missing files, malformed JSON, and version mismatches while enforcing the read cap.
• Add a regression test TestLoadRuleStatsHistoryRejectsOversizedFile in tests/support/rule_stats_history_test.go that writes a file larger than the 32 MiB cap and asserts the loader returns empty history.

Testing

• go test ./tests/support succeeded.
• go test ./internal/codeguard/runner/support succeeded.
• go test ./tests/cli was executed together with the support tests and succeeded (tests reported OK/cached).
• git diff --check and repository status checks completed with no issues.

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Codex Task