sql,hints: improve EXPLAIN output for statement hints

[DrewKimball]

sql: show applied vs total count for statement hints in EXPLAIN

Track runtime hint errors (planning fallbacks, session variable
failures, validation errors) on the instrumentation helper so that
EXPLAIN output correctly reflects which hints were applied vs skipped.

The hint model is refactored from a boolean Enabled field to an
Err error field with sentinel errors for disabled and duplicate
hints. Runtime errors are stored separately in a runtimeHintErrors
map to avoid mutating shared hint state across prepared statement
executions.

Informs: #163878

Release note (sql change): EXPLAIN and EXPLAIN ANALYZE now display
statement hint counts as "applied" versus "skipped" rather than a
simple total. Hints that fail at runtime (e.g. referencing a
non-existent index or an invalid session variable) are correctly
reflected as skipped.

Co-Authored-By: roachdev-claude roachdev-claude-bot@cockroachlabs.com

sql: add verbose EXPLAIN output for statement hint details

When EXPLAIN (VERBOSE) or EXPLAIN ANALYZE (VERBOSE) is used with
statement hints, emit a tree showing each hint's type, details (donor
SQL or variable name/value), and skip reason when applicable. Hints
are grouped into "applied" and "skipped" sections.

Fixes: #163878

Release note (sql change): EXPLAIN (VERBOSE) and EXPLAIN ANALYZE
(VERBOSE) now include a detailed tree of statement hints showing
each hint's type, configuration (donor SQL or variable name/value),
and skip reason when applicable.

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This change is 

[ZhouXing19]

I only have nit comments, and defer the final stamp to Michael.

@ZhouXing19 reviewed 12 files and all commit messages, and made 4 comments.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on DrewKimball and michae2).

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pkg/sql/plan_opt.go line 77 at r1 (raw file):

		// Do not return the error. If semantic analysis failed, try preparing again
		// without injected hints.
		log.Eventf(ctx, "preparing with injected hints failed with: %v", err)

Should we do p.instrumentation.recordHintError() here as well?

---

pkg/sql/opt/exec/explain/output.go line 350 at r1 (raw file):

	} else {
		skipped := total - applied
		ob.AddTopLevelField("statement hints", fmt.Sprintf(

Nice addition!

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pkg/sql/opt/exec/execbuilder/testdata/statement_hint_builtins line 2295 at r2 (raw file):

statement ok
EXECUTE hint_retry_stmt

nit: might be more informative to run it with EXPLAIN ANALYZE (VERBOSE) EXECUTE.