feat: add opt-in JSONL format for trace logs (Closes #8066)

[Pleiades1726]

背景

详见 Issue #8066。当前 trace 日志格式为带 [time] 前缀的格式，导致每行不是合法的 JSON，下游工具（jq、Python json.loads、ELK 等）无法直接解析。

改动

新增配置项 trace_log_format，支持两种模式：

值	说明	示例
prefixed（默认）	保持现有格式，向后兼容	[2026-05-07 13:45:12.123] {...}
jsonl	纯 JSON Lines，无前缀	{...}

修改文件

• astrbot/core/config/default.py — 添加默认值 + metadata schema（两处）
• astrbot/core/log.py — _add_file_sink() 新增 trace_format 参数，configure_trace_logger() 读取配置并传递

使用方式

在 data/config.yaml 中设置：

trace_log_format: jsonl

重启后 trace 日志将输出纯 JSON Lines 格式。

Summary by Sourcery

Add a configurable format for trace log output, allowing opt-in JSON Lines in addition to the existing prefixed format.

New Features:

• Introduce the trace_log_format configuration option to select between prefixed and JSONL trace log formats.

Enhancements:

• Extend trace logger setup to honor the configured trace_log_format while preserving current behavior as the default.

[sourcery-ai]

Hey - I've left some high level feedback:

• When trace_log_format has an unexpected value, _add_file_sink silently falls back to the prefixed format; consider validating against the allowed options and either raising or logging a warning so misconfigurations are visible.
• You now have the literal strings "prefixed" and "jsonl" in both the config schema and log.py; consider centralizing these as constants or an enum-like structure to avoid divergence if new formats are added later.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- When `trace_log_format` has an unexpected value, `_add_file_sink` silently falls back to the prefixed format; consider validating against the allowed options and either raising or logging a warning so misconfigurations are visible.
- You now have the literal strings "prefixed" and "jsonl" in both the config schema and `log.py`; consider centralizing these as constants or an enum-like structure to avoid divergence if new formats are added later.

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[gemini-code-assist]

Code Review

This pull request introduces a new configuration option trace_log_format for trace logging, allowing users to choose between a timestamp-prefixed format and a raw jsonl format. The changes include updates to the default configuration, schema validation, and the logging utility to handle the format selection. I have no feedback to provide.