Replace N+1 execution fetches in batch-status handler with one batch fetch

[AbirAbbas]

Summary

• Replaces N+1 per-ID storage fetches in handleBatchStatus with a single GetExecutionRecordsBatch query using a parameterized WHERE execution_id IN (...) clause.
• Preserves the per-ID not_found response contract for missing execution IDs while collapsing all DB round-trips to one.
• Rejects oversized batches (>500 IDs, 400 on the oversize path) with a 400 before any storage call, and reuses the batched webhook-registration lookup to avoid a secondary N+1.

Changes

• control-plane/internal/storage/storage.go: adds GetExecutionRecordsBatch to the StorageProvider/ExecutionStore interfaces.
• control-plane/internal/storage/execution_records.go: implements GetExecutionRecordsBatch on LocalStorage with a single parameterized IN (...) query.
• control-plane/internal/handlers/execute.go: rewrites handleBatchStatus to fetch once and enforce the batch-size cap.
• Test stubs across internal/handlers/* and internal/server/* updated to satisfy the new storage interface; new execute_batch_status_test.go and execution_records_test.go cover the batch path, per-ID not_found mapping, and oversize rejection.

Test plan

• cd control-plane && go vet ./... (clean).
• cd control-plane && go test ./internal/handlers/... ./internal/storage/... (passing).
• Confirm a 500-ID batch triggers exactly one storage call and that oversize batches (e.g. 501 IDs) return 400 without hitting storage.

Closes #430

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[github-actions]

📊 Coverage gate

Thresholds from .coverage-gate.toml: per-surface ≥ 84%, aggregate ≥ 85%, max per-surface regression ≤ 1.0 pp, max aggregate regression ≤ 0.50 pp.

Surface	Current	Baseline	Δ
control-plane	86.90%	87.40%	↓ -0.50 pp	🟡
sdk-go	92.40%	92.00%	↑ +0.40 pp	🟢
sdk-python	93.82%	93.73%	↑ +0.09 pp	🟢
sdk-typescript	91.08%	90.42%	↑ +0.66 pp	🟢
web-ui	84.76%	84.79%	↓ -0.03 pp	🟡
aggregate	85.55%	85.75%	↓ -0.20 pp	🟡

✅ Gate passed

No surface regressed past the allowed threshold and the aggregate stayed above the floor.

[github-actions]

📐 Patch coverage gate

Threshold: 80% on lines this PR touches vs origin/main (from .coverage-gate.toml:thresholds.min_patch).

Surface	Touched lines	Patch coverage	Status
control-plane	61	90.00%	✅
sdk-go	0	—	➖ no changes
sdk-python	0	—	➖ no changes
sdk-typescript	0	—	➖ no changes
web-ui	0	—	➖ no changes

✅ Patch gate passed

Every surface whose lines were touched by this PR has patch coverage at or above the threshold.