[MCP] Redact environment variable values in /mcp and mcp get (#5648)

Fixes #5524

codex-rs/tui/src/snapshots/codex_tui__history_cell__tests__mcp_tools_output_masks_sensitive_values.snap

@@ -0,0 +1,27 @@
+---
+source: tui/src/history_cell.rs
+assertion_line: 1540
+expression: rendered
+---
+/mcp
+
+🔌  MCP Tools
+
+  • docs
+    • Status: enabled
+    • Auth: Unsupported
+    • Command: docs-server
+    • Env: TOKEN=*****, APP_TOKEN=*****
+    • Tools: list
+    • Resources: (none)
+    • Resource templates: (none)
+
+  • http
+    • Status: enabled
+    • Auth: Unsupported
+    • URL: https://example.com/mcp
+    • HTTP headers: Authorization=*****
+    • Env HTTP headers: X-API-Key=API_KEY_ENV
+    • Tools: ping
+    • Resources: (none)
+    • Resource templates: (none)