fix: correct state transitions in filter sync manager

[xdustinface]

Move set_state(Syncing) after the early returns in start_download() so the manager stays in the correct state when filter headers haven't reached the needed height. Return to WaitForEvents in that case so new FilterHeadersStored events re-trigger start_download with proper batch processing initialization.

Also transition Synced back to Syncing in handle_new_filter_headers when new filter headers arrive, ensuring is_synced() returns false until all new filters and matched blocks are fully processed.

Based on:

• refactor: simplify FiltersManager.start_download #450

Summary by CodeRabbit

• Bug Fixes
  • Improved filter synchronization reliability during initial download and state transitions
  • Enhanced handling of filter header updates to ensure proper synchronization state management

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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📝 Walkthrough

Walkthrough

Refactored state transitions in the filter manager's download and event handling logic. Removes immediate Syncing transition, introduces WaitForEvents state for insufficient headers, defers transitions based on available data, and enhances lookahead batch creation. Tests expanded to validate new state behavior and transitions.

Changes

Cohort / File(s)	Summary
Filter Manager State Machine dash-spv/src/sync/filters/manager.rs	Reworked state transitions: removed immediate Syncing at start_download, added WaitForEvents state when headers insufficient, explicit transition to Syncing after computing download_start, Synced→Syncing transition on new headers, and conditional lookahead batch creation. Test imports and validations for new state flows added.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~15 minutes

Poem

🐰 State machines dance in careful measure,
Headers waiting, transitions at leisure,
WaitForEvents guards the gate,
Until filters sync, filters sync with fate,
Lookahead batches bloom so bright! ✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'fix: correct state transitions in filter sync manager' directly and clearly describes the main change: fixing state transitions in the filter sync manager, which is the core purpose of this PR.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/filters-state-transitions

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[github-actions]

This PR has merge conflicts with the base branch. Please rebase or merge the base branch into your branch to resolve them.