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.

tests/tools/test_mcp_probe.py

@@ -61,7 +61,8 @@ class TestProbeMcpServerTools:
         async def fake_connect(name, cfg):
             return mock_server
 
-        with patch("tools.mcp_tool._load_mcp_config", return_value=config), \
+        with patch("tools.mcp_tool._MCP_AVAILABLE", True), \
+             patch("tools.mcp_tool._load_mcp_config", return_value=config), \
              patch("tools.mcp_tool._connect_server", side_effect=fake_connect), \
              patch("tools.mcp_tool._ensure_mcp_loop"), \
              patch("tools.mcp_tool._run_on_mcp_loop") as mock_run, \
@@ -102,7 +103,8 @@ class TestProbeMcpServerTools:
                 raise ConnectionError("Server not found")
             return mock_server
 
-        with patch("tools.mcp_tool._load_mcp_config", return_value=config), \
+        with patch("tools.mcp_tool._MCP_AVAILABLE", True), \
+             patch("tools.mcp_tool._load_mcp_config", return_value=config), \
              patch("tools.mcp_tool._connect_server", side_effect=fake_connect), \
              patch("tools.mcp_tool._ensure_mcp_loop"), \
              patch("tools.mcp_tool._run_on_mcp_loop") as mock_run, \
@@ -135,7 +137,8 @@ class TestProbeMcpServerTools:
         async def fake_connect(name, cfg):
             return mock_server
 
-        with patch("tools.mcp_tool._load_mcp_config", return_value=config), \
+        with patch("tools.mcp_tool._MCP_AVAILABLE", True), \
+             patch("tools.mcp_tool._load_mcp_config", return_value=config), \
              patch("tools.mcp_tool._connect_server", side_effect=fake_connect), \
              patch("tools.mcp_tool._ensure_mcp_loop"), \
              patch("tools.mcp_tool._run_on_mcp_loop") as mock_run, \
@@ -159,7 +162,8 @@ class TestProbeMcpServerTools:
         """_stop_mcp_loop is called even when probe fails."""
         config = {"github": {"command": "npx", "connect_timeout": 5}}
 
-        with patch("tools.mcp_tool._load_mcp_config", return_value=config), \
+        with patch("tools.mcp_tool._MCP_AVAILABLE", True), \
+             patch("tools.mcp_tool._load_mcp_config", return_value=config), \
              patch("tools.mcp_tool._ensure_mcp_loop"), \
              patch("tools.mcp_tool._run_on_mcp_loop", side_effect=RuntimeError("boom")), \
              patch("tools.mcp_tool._stop_mcp_loop") as mock_stop:
@@ -187,7 +191,8 @@ class TestProbeMcpServerTools:
             connect_calls.append(name)
             return mock_server
 
-        with patch("tools.mcp_tool._load_mcp_config", return_value=config), \
+        with patch("tools.mcp_tool._MCP_AVAILABLE", True), \
+             patch("tools.mcp_tool._load_mcp_config", return_value=config), \
              patch("tools.mcp_tool._connect_server", side_effect=fake_connect), \
              patch("tools.mcp_tool._ensure_mcp_loop"), \
              patch("tools.mcp_tool._run_on_mcp_loop") as mock_run, \