Add usage event tracking for explain error requests

[shenxianpeng]

Summary

• add UsageEvent, UsageRecorder, and UsageRecorders to capture Explain Error request outcomes
• record usage for pipeline and console flows, including success, cache hits, disabled, misconfigured, and provider error cases
• persist provider model and input log line counts on saved explanations and update tests for tracking and invalid provider behavior
• ignore .gstack in Git

Testing

• Not run (not requested)

close #130

[Copilot]

Pull request overview

Adds an internal usage-event model and recorder plumbing to track Explain Error request outcomes across both pipeline-step and console-action entry points, and persists additional provider metadata on saved explanations.

Changes:

• Introduce UsageEvent, UsageRecorder, and UsageRecorders for internal usage tracking.
• Emit usage events from pipeline and console flows (success, cache hit, disabled, misconfigured, provider error).
• Persist provider model + input log line count on ErrorExplanationAction, and update tests accordingly.

Reviewed changes

Copilot reviewed 8 out of 9 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/main/java/io/jenkins/plugins/explain_error/UsageEvent.java	Adds immutable usage event record + enums for entry point/result.
src/main/java/io/jenkins/plugins/explain_error/UsageRecorder.java	Adds extension point interface for recording usage events.
src/main/java/io/jenkins/plugins/explain_error/UsageRecorders.java	Resolves recorders via ExtensionList and dispatches events safely.
src/main/java/io/jenkins/plugins/explain_error/ErrorExplainer.java	Emits usage events from the shared pipeline + console explanation paths.
src/main/java/io/jenkins/plugins/explain_error/ConsoleExplainErrorAction.java	Emits cache-hit/disabled usage events for console-triggered requests.
src/main/java/io/jenkins/plugins/explain_error/ErrorExplanationAction.java	Persists provider model + input log line count for later cache-hit attribution.
src/test/java/io/jenkins/plugins/explain_error/UsageTrackingTest.java	New tests validating usage events for pipeline + console flows.
src/test/java/io/jenkins/plugins/explain_error/ExplainErrorStepTest.java	Updates expectations for invalid-provider logging behavior.
.gitignore	Ignores .gstack.

[Copilot]

When explanation is disabled, the usage event is recorded with a null provider (and therefore "Unknown" provider/model). If the provider is configured but disabled by config, it’s still useful for tracking to include the configured provider name/model; consider resolving the provider (or reading it from the effective config) before recording the DISABLED event.