fix: remove stale test skips, fix regex backtracking, file search bug, and test flakiness

Bug fixes:
- agent/redact.py: catastrophic regex backtracking in _ENV_ASSIGN_RE — removed
  re.IGNORECASE and changed [A-Z_]* to [A-Z0-9_]* to restrict matching to actual
  env var name chars. Without this, the pattern backtracks exponentially on large
  strings (e.g. 100K tool output), causing test_file_read_guards to time out.
- tools/file_operations.py: over-escaped newline in find -printf format string
  produced literal backslash-n instead of a real newline, breaking file search
  result parsing (total_count always 1, paths concatenated).

Test fixes:
- Remove stale pytestmark.skip from 4 test modules that were blanket-skipped as
  'Hangs in non-interactive environments' but actually run fine:
  - test_413_compression.py (12 tests, 25s)
  - test_file_tools_live.py (71 tests, 24s)
  - test_code_execution.py (61 tests, 99s)
  - test_agent_loop_tool_calling.py (has proper OPENROUTER_API_KEY skip already)
- test_413_compression.py: fix threshold values in 2 preflight compression tests
  where context_length was too small for the compressed output to fit in one pass.
- test_mcp_probe.py: add missing _MCP_AVAILABLE mock so tests work without MCP SDK.
- test_mcp_tool_issue_948.py: inject MCP symbols (StdioServerParameters etc.) when
  SDK is not installed so patch() targets exist.
- test_approve_deny_commands.py: replace time.sleep(0.3) with deterministic polling
  of _gateway_queues — fixes race condition where resolve fires before threads
  register their approval entries, causing the test to hang indefinitely.

Net effect: +256 tests recovered from skip, 8 real failures fixed.

agent/redact.py

@@ -53,8 +53,7 @@ _PREFIX_PATTERNS = [
 # ENV assignment patterns: KEY=value where KEY contains a secret-like name
 _SECRET_ENV_NAMES = r"(?:API_?KEY|TOKEN|SECRET|PASSWORD|PASSWD|CREDENTIAL|AUTH)"
 _ENV_ASSIGN_RE = re.compile(
-    rf"([A-Z_]{{0,50}}{_SECRET_ENV_NAMES}[A-Z_]{{0,50}})\s*=\s*(['\"]?)(\S+)\2",
-    re.IGNORECASE,
+    rf"([A-Z0-9_]{{0,50}}{_SECRET_ENV_NAMES}[A-Z0-9_]{{0,50}})\s*=\s*(['\"]?)(\S+)\2",
 )
 
 # JSON field patterns: "apiKey": "value", "token": "value", etc.