refactor(query-engine): remove is_exact_query as a threaded bool

[milindsrivastava1997]

What

StoreQueryParams::is_exact_query was a bool computed once and threaded through StoreQueryPlan, then consulted by execute_store_query to pick between store fetch strategies. Every StoreQueryParams builder had to independently derive it correctly, and past call sites already drifted out of sync more than once (#580, #582, #608).

Since #616 replaced the tolerant-scan branch with scan_windows_via_exact (a grid-walk of exact lookups), the flag's only remaining job was choosing between that grid-walk and a single direct exact call — but a range exactly one window wide already makes scan_windows_via_exact degenerate to a single exact lookup. So the flag goes away entirely:

• create_store_query_plan still narrows the Sliding-instant values query to one window's width (unchanged math), but now returns WindowType alongside the plan instead of stashing a derived bool on the struct.
• create_keys_query_params and finish_range_context no longer set anything — the "always grid-walk" behavior for keys/range falls out naturally from execute_store_query always calling scan_windows_via_exact, with nothing left to drift out of sync.
• execute_store_query collapsed to a thin wrapper that always calls scan_windows_via_exact.
• execute_and_merge_store_queries's Sliding-vs-Tumbling merge decision (unrelated to fetch mechanism) now takes WindowType as an explicit parameter instead of re-deriving it from the removed field.
• QueryExecutionContext gained a value_window_type field, threaded through all three query-language front-ends (PromQL, Elastic, SQL).

Testing

Added window_semantics_consistency_tests.rs: hardening tests written against the observable PromQL query surface (not internal struct/function names) so they hold regardless of internal reshaping — covering Sliding/Tumbling instant-vs-range agreement (including the historical overlap-summing bug shape), keys queries over each WindowType, and window-grid boundary cases (exact-width vs narrower-than-bucket spans).

cargo check --tests, cargo clippy -- -D warnings, and cargo fmt --check all clean. Full suite: 505/505 passing, zero regressions.

Fixes #613.

🤖 Generated with Claude Code