fix(tests): mock redact_sensitive_text in large-content test cases

[dsr-restyn]

Problem

redact_sensitive_text runs a multi-pass regex scan that is O(n) but slow enough on 100K+ char inputs to exceed the 30s per-test signal.alarm in conftest.py. This caused three tests in test_file_read_guards.py to return {'error': 'Test exceeded 30 second timeout'} as a JSON result instead of the expected response — tests appeared to pass in pytest but produced wrong output.

Fix

Mock redact_sensitive_text as a no-op in the three affected tests. These tests exercise the character-count guard and config override logic, not redaction behaviour — bypassing redaction is correct here.

Affected tests

• TestCharacterCountGuard::test_oversized_read_rejected — 100,001 chars through redact before the guard rejects it
• TestCharacterCountGuard::test_content_under_limit_passes — 99,999 chars through redact
• TestConfigOverride::test_custom_config_raises_limit — 200,000 chars through redact

Verification

pytest tests/tools/test_file_read_guards.py::TestCharacterCountGuard tests/tools/test_file_read_guards.py::TestConfigOverride -v
5 passed in 0.70s

Stacks on top of #fix/block-proc-environ-read → upstream PR NousResearch#4609.

[dsr-restyn]

Superseded by upstream PR NousResearch#4860 which targets main directly. The fix will land in the fix/block-proc-environ-read branch naturally once NousResearch#4860 merges.