Scope nullable block guard detection to the matched guard body

[alxxjohn]

Motivation

• The previous block-style null-guard helper scanned a fixed 3,000-byte window after if (!name) { and could credit an exit keyword that occurred outside the guard block, producing false negatives for defensive.null-assumption.
• The intent is to ensure only exits that actually occur inside the matched null-check block suppress null-dereference findings while preserving nested-block support.

Description

• Replace the 3,000-byte window scan in nullableParamHasBlockExitGuard with brace-depth tracking so the function locates the matching } and only searches for return|throw|continue|break inside that exact block in internal/codeguard/checks/quality/quality_defensive.go.
• Preserve the existing single-line guard detection and the other guard heuristics in nullableParamGuarded while making the block-style check conservative for unclosed blocks.
• Add focused regression tests in internal/codeguard/checks/quality/quality_defensive_test.go that validate exits inside the guard, exits after the guard (should not count), nested guard exits, and unclosed guard behavior.

Testing

• Ran go test ./internal/codeguard/checks/quality ./tests/checks and the test suites completed successfully, including the new regression test file (tests passed).
• Executed go test ./internal/codeguard/checks/quality to verify the package-level tests and the new unit tests, which passed.
• Noted the repository szr wrapper was recommended by local AGENTS.md but was not available in the execution environment, so tests were run with the standard go test command.

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Codex Task