feat: enable /logs API endpoint in standalone (Docker CE) mode

[ilopezluna]

When MODEL_RUNNER_LOG_DIR is not set, auto-create a default log directory /tmp/model-runner-logs and set up tee logging so that all output goes to both stderr and bracket- timestamped log files (for the /logs API endpoint).

Changes:

• Add BracketWriter to pkg/logging that prefixes each line with [timestamp] in the format expected by MergeLogs
• Auto-create log directory and open service + engine log files
• Use io.MultiWriter to tee slog output to stderr and log files
• Configure ServerLogFactory for engine backend logs
• Always enable /logs endpoint when a log directory is available

[gemini-code-assist]

Code Review

This pull request enables the /logs API endpoint across all deployment modes by automatically creating a default log directory and implementing file-backed logging using a new BracketWriter. The changes improve operational visibility and debugging capabilities, supported by new unit tests and goroutine leak detection. A critical issue was identified in pkg/logging/logging.go at lines 117-123, where the BracketWriter.Write method risks data loss by clearing its internal buffer before confirming a successful write to the underlying writer; the buffer must only be advanced after the write succeeds.

[sourcery-ai]

Hey - I've left some high level feedback:

• The new LogDir() comment suggests the server auto-creates a default directory, but this behavior actually lives in main; consider updating the comment (or moving the logic) so the function comment accurately reflects its behavior.
• BracketWriter currently buffers until a newline is seen and never flushes partial lines, which can cause unbounded memory growth and dropped logs on process exit; consider adding a Flush/Close path or a max-buffer safeguard.
• Engine logging uses a shared *os.File with separate BracketWriters created per backend; if multiple backends write concurrently, their prefix+line writes to the shared file may interleave—consider either sharing a single BracketWriter per file or serializing access at a higher level.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The new LogDir() comment suggests the server auto-creates a default directory, but this behavior actually lives in main; consider updating the comment (or moving the logic) so the function comment accurately reflects its behavior.
- BracketWriter currently buffers until a newline is seen and never flushes partial lines, which can cause unbounded memory growth and dropped logs on process exit; consider adding a Flush/Close path or a max-buffer safeguard.
- Engine logging uses a shared *os.File with separate BracketWriters created per backend; if multiple backends write concurrently, their prefix+line writes to the shared file may interleave—consider either sharing a single BracketWriter per file or serializing access at a higher level.

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