fix(reliability): make goroutine-in-loop detection linear to avoid quadratic AST walks

[alxxjohn]

Motivation

• The previous implementation called isInsideLoop (which performed a full ast.Inspect over the function body) for every *ast.GoStmt, producing quadratic behavior and allowing attacker-controlled source to cause excessive CPU usage during reliability scans.
• Reliability checks are enabled by some default profiles, so this path is reachable in hardened CI and needed a linear-time fix to prevent CI DoS.

Description

• Replace repeated full-function traversals with loop nesting tracking performed during the existing ast.Inspect pass by adding loopDepth and loopStack and checking loopDepth > 0 for goroutine-in-loop classification.
• Remove the expensive helper isInsideLoop from reliability_go_helpers.go and update functionReliabilityFindings in reliability_go.go to use the in-pass loop depth check instead of extra walks.
• Preserve existing rule semantics and finding generation (reliability.unbounded-work and related checks) while changing only the detection mechanism to be linear in AST size.

Testing

• Ran formatting with gofmt -w internal/codeguard/checks/reliability/*.go and it completed successfully.
• Executed go test ./tests/checks and the test suite ran successfully in this environment.
• Executed go test ./internal/codeguard/checks/reliability which reported there are no test files for that package, so no package-local tests were run.
• Ran git diff --check to ensure no whitespace or trivial issues and it reported no problems.

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Codex Task