* fix(config): pin explicit app config path across get_app_config
Fixes#7086. reload_app_config(config_path) now records an
_app_config_source pin (mirroring _extensions_config_source) so
get_app_config and DeerFlowClient(config_path=...) keep following the
explicit file instead of ambient default resolution.
* fix(config): synchronize app config cache and absolute pins
Serialize singleton reads, reloads, resets and injections so a concurrent call cannot publish a config under another file's pin. Keep explicit paths absolute across working-directory changes while preserving ambient discovery and failed-reload behavior.
Add deterministic concurrency and relative-path regressions. Validation: 33,875 offline tests and 262 blocking-I/O tests passed; frontend check, backend formatting and agent guidance passed.
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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
