[upstream-sync] Component run_in_background() error callback (upstream PR #1754)

[github-actions]

Upstream Change Summary

Type: new-feature
Difficulty: Medium
Recommendation: Adapt

Upstream PR #1754 adds scope-level and environment-level exception handlers for background mount() failures. The Rust core change is in rust/core/src/engine/component.rs: Component::run_in_background() now accepts an optional async on_error callback instead of unconditionally logging errors. This is a meaningful API extension to the component execution engine.

The broader Python-side exception handler chaining (ExceptionContext, ExceptionHandler, coco.exception_handler()) is irrelevant to recoco, but the Rust signature change to run_in_background() may affect recoco's component API and background execution model.

Upstream References

• PR: Feat: Mount exception handler cocoindex-io/cocoindex#1754
• Upstream repo: https://github.com/cocoindex-io/cocoindex

Relevant Upstream Files / Areas

• rust/core/src/engine/component.rs → crates/recoco-core/src/execution/ component module
• rust/py/src/component.rs → skip (PyO3 bindings)
• All other files (python/, docs/) → skip

Recoco Considerations

• Affected modules: crates/recoco-core component execution, specifically run_in_background()
• Python exclusions: Do not port ExceptionContext, ExceptionHandler, coco.exception_handler(), EnvironmentBuilder.set_exception_handler(), or any Python API surface
• Adaptation needed: The run_in_background() signature change should be adopted in pure-Rust form — likely as an optional on_error: Option<impl FnOnce(Error) + Send + 'static> callback or a more idiomatic Rust error-channel approach
• API surface impact: If run_in_background() or background component execution is exposed in recoco's public API, this may require a minor version bump
• Feature-gating: No new ops, no new feature gate needed
• Opportunity: Consider whether recoco should expose a richer error-handling API for background flows (e.g., returning a JoinHandle or using a broadcast channel for errors) rather than mirroring the callback pattern

Integration Notes

The key question is whether recoco's run_in_background() equivalent currently swallows errors or propagates them. If it already returns a JoinHandle<Result<...>>, this upstream change may be partially covered. Review the current implementation before adopting. The Python-specific handler chaining logic should be entirely omitted.