📝 Add configured logger file state API docs

docs/api/configured-logger-file-open-failures.md

@@ -0,0 +1,74 @@
+---
+name: configured-logger-file-open-failures
+group: api
+category: runtime
+update-time: 20260512
+description: Read the number of open failures recorded by the configured runtime file sink.
+key-word:
+    - logger
+    - runtime
+    - file
+    - public
+---
+
+## Configured-logger-file-open-failures
+
+Read the number of open failures recorded by a `ConfiguredLogger` file sink. This helper is useful for diagnosing file-availability or reopen problems.
+
+### Interface
+
+```moonbit
+pub fn ConfiguredLogger::file_open_failures(self : ConfiguredLogger) -> Int {}
+```
+
+#### input
+
+- `self : ConfiguredLogger` - Config-driven runtime logger whose open-failure counter should be inspected.
+
+#### output
+
+- `Int` - Number of recorded open failures.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- File-backed sinks report their recorded open-failure count.
+- Queued file sinks forward the metric from the wrapped file sink.
+- Non-file sinks return `0`.
+- The counter is cumulative until reset.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need File Availability Diagnostics
+
+When operators should see whether file open or reopen failed:
+```moonbit
+let count = logger.file_open_failures()
+```
+
+In this example, the configured logger exposes the open-failure metric directly.
+
+#### When Validate Recovery Logic
+
+When tests or support code should inspect whether reopen attempts failed:
+```moonbit
+ignore(logger.file_open_failures())
+```
+
+In this example, the counter helps confirm whether file-open problems occurred during runtime.
+
+### Error Case
+
+e.g.:
+- If the configured sink is not file-backed, the method returns `0`.
+
+- If callers need the full file status snapshot rather than one counter, `file_state()` is the better API.
+
+### Notes
+
+1. Use this helper for focused file-open diagnostics.
+
+2. Pair it with `file_reopen(...)` and `file_available()` during recovery analysis.