refactor: reduce complexity of validate_pr_filters/validate_pipeline_filters in filter_ir.rs

[github-actions]

What was complex

validate_pr_filters (112 lines) and validate_pipeline_filters (46 lines) in src/compile/filter_ir.rs both contained:

1. Copy-pasted time-window block — the identical 3-check sequence (start format, end format, start == end) appeared verbatim in both functions.
2. Repeated include/exclude overlap pattern — the find_overlap + if !overlap.is_empty() { diags.push(...) } idiom appeared inline 3 times in validate_pr_filters (for author, build-reason, and twice in labels).
3. Inline labels block — 3 overlap/empty-check sub-cases nested directly inside validate_pr_filters.

What changed

Extracted three private helpers, keeping public API signatures unchanged:

Helper	Purpose
validate_time_window	Shared HH:MM format + zero-width window checks; eliminates the copy-paste between the two public functions
check_include_exclude_overlap	Single place for the include/exclude conflict pattern, parametric on filter name
validate_labels_filter	Encapsulates the three labels sub-checks (any-of∩none-of, all-of∩none-of, empty warning)

Before / After

Function	Before	After
validate_pr_filters	112 lines, 4 repeated inline patterns, nesting depth 4	~30 lines, delegates to helpers
validate_pipeline_filters	46 lines, copy-pasted time-window block	~12 lines

Verification

• cargo build — clean
• cargo test — all 2580+ tests pass, 0 failures
• Behaviour is identical — all helpers are pure extractions of existing logic

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