refactor(storage): make remaining sqlstore SQL dialect-portable

[Kiran01bm]

Summary

Removes the last MySQL-only SQL constructs from pkg/storage/internal/sqlstore by moving them behind the storage Dialect seam or rewriting them portably. No behavior change for MySQL.

Why

The sqlstore is being decoupled from MySQL so a second storage dialect can implement the same interface without rewriting query call sites. These were the remaining hard spots where MySQL syntax leaked into shared query text; each is now either dialect-rendered or genuinely portable SQL.

What

• INSERT IGNORE → Dialect.InsertIfAbsent(conflictColumns), which returns the modifier/suffix fragments around a portable INSERT INTO ... VALUES. The contract requires 1 affected row on insert and 0 on conflict, so callers keep their existing affected-rows checks.
• TIMESTAMPDIFF(MICROSECOND, ...) in webhook inbox stats → select MIN(received_at) and the database's current timestamp, compute the age in Go.
• SUBSTRING_INDEX → portable POSITION/SUBSTRING/CASE expression.
• JSON boolean predicate (JSON_EXTRACT ... <=> CAST('true' AS JSON)) → Dialect.JSONBooleanIsTrue(expr, path), defined to yield false for missing paths, JSON null, SQL NULL, and non-boolean values. Path keys must be plain identifiers; implementations panic otherwise.
• FORCE INDEX in the active-apply target check → Dialect.IndexHint(index); dialects without hint syntax return an empty string.

Before / after

Before                                     After
┌───────────────────────────────┐          ┌───────────────────────────────┐
│ shared query text contains:   │          │ shared query text is portable │
│  INSERT IGNORE                │          │ SQL; dialect renders:         │
│  TIMESTAMPDIFF(...)           │  ──────▶ │  InsertIfAbsent fragments     │
│  SUBSTRING_INDEX(...)         │          │  JSONBooleanIsTrue predicate  │
│  JSON_EXTRACT <=> CAST(...)   │          │  IndexHint text               │
│  FORCE INDEX (...)            │          │ (durations computed in Go)    │
└───────────────────────────────┘          └───────────────────────────────┘

[Copilot]

Pull request overview

This PR refactors pkg/storage/internal/sqlstore to remove remaining MySQL-specific SQL constructs by routing them through the Dialect interface or rewriting them using more portable SQL patterns, with the intent of enabling additional storage backends without rewriting query call sites.

Changes:

• Added new Dialect seams for InsertIfAbsent, JSONBooleanIsTrue, and IndexHint, and wired call sites to use them.
• Replaced MySQL-only SQL in query text (e.g., TIMESTAMPDIFF, SUBSTRING_INDEX, FORCE INDEX, JSON boolean comparisons) with portable SQL and/or Go-side computation.
• Updated store wiring so applyCommentStore uses the injected dialect rather than a hard-coded MySQL dialect.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
pkg/storage/internal/sqlstore/webhook_events.go	Replaces TIMESTAMPDIFF-based age computation with portable selection + Go duration calculation.
pkg/storage/internal/sqlstore/storage.go	Wires applyCommentStore to use deps.Dialect (dialect seam) instead of MySQLDialect{}.
pkg/storage/internal/sqlstore/dialect.go	Extends Dialect and implements MySQL versions of InsertIfAbsent, JSONBooleanIsTrue, and IndexHint.
pkg/storage/internal/sqlstore/dialect_test.go	Adds unit tests for the new MySQL dialect methods.
pkg/storage/internal/sqlstore/apply_operations.go	Replaces SUBSTRING_INDEX and moves JSON boolean predicate behind Dialect.JSONBooleanIsTrue.
pkg/storage/internal/sqlstore/apply_comments.go	Replaces INSERT IGNORE with Dialect.InsertIfAbsent(...) fragments.
pkg/storage/internal/sqlstore/applies.go	Replaces FORCE INDEX with Dialect.IndexHint(...) and threads dialect through target-lock checks/claim paths.

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[morgo]

🤖 Approving on Morgan's behalf (automated review, escalation rules apply).

Checked each rewrite for MySQL equivalence:

• SUBSTRING_INDEX(k,'/',1) → CASE WHEN POSITION... THEN k ELSE SUBSTRING(...): equivalent including the no-slash (whole key) and leading-slash (empty prefix) edges; the new slash-less finalizer integration test covers the branch that matters.
• defer_cutover gate: JSONBooleanIsTrue renders the same null-safe <=> CAST('true' AS JSON) predicate modulo the quoted path ($."defer_cutover"), which MySQL treats identically; unit test pins the exact SQL.
• InsertIfAbsent: renders the same INSERT IGNORE, and the affected-rows contract (1 on insert, 0 on conflict) matches how ClaimSummaryComment decides the winner.
• IndexHint: same FORCE INDEX, now backtick-escaped.
• InboxStats: age subtraction moves into Go with the same NULL/zero handling — metrics-only path.

Non-blocking note: #1006 edits the adjacent storage.go constructor lines (planComments/settings vs. applyComments here), so whichever merges second will likely need a trivial rebase.