Fix multiline expression index handling

[Pybsama]

Description

pg_get_indexdef can preserve newlines inside expression index definitions. The existing anchored rewrite uses .*, which does not match newlines in Go, so converting a valid definition to CREATE INDEX CONCURRENTLY fails. The same conversion is also used while validating plans with concurrent index operations disabled.

This change enables dot-all matching only for the existing regex, preserving its anchors, case sensitivity, and exact CREATE [UNIQUE] INDEX prefix. It adds regression coverage for:

• the direct statement conversion;
• adding a multiline expression index through the default concurrent path;
• validating a migration with an existing multiline expression index under WithNoConcurrentIndexOps().

Testing

• GOTOOLCHAIN=go1.25.5 go test -race ./... -timeout 30m
• focused conversion test, 100 repetitions
• both PostgreSQL 17.10 acceptance scenarios, 10 repetitions
• golangci-lint v2.7.0 run
• go vet ./...
• sqlfluff 3.3.0 lint
• gofmt, go mod tidy -diff, and git diff --check

The commit is GitHub Verified.

Fixes #253

[cla-assistant]

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[Pybsama]

Revalidated this change against current main at 9ada4710c28718312557c84cca1be4b2557eed54. The one commit since the PR base only changes materialized-view handling; the PR applies without conflicts, and the exact current-main merge tree is 6e3c23613b66d86b32aae0f226f2a7fefadaab50.

Current-main validation:

• restoring the old single-line regex makes all three regressions fail independently: direct conversion, default concurrent index creation, and WithNoConcurrentIndexOps() plan validation
• restoring (?s) makes all three pass
• Go 1.25.5: full go test -race ./... -timeout 30m passed; the acceptance package completed in 145.747s
• focused conversion: 100/100 repetitions
• both PostgreSQL 17.10 acceptance scenarios: 10/10 repetitions
• golangci-lint 2.7.0 with Go 1.25.5: 0 issues; go vet ./... passed
• sqlfluff 3.3.0: passed; gofmt, go mod tidy -diff, and git diff --check: clean
• the multiline-string dry-run still names only the two unchanged baseline files (policy_cases_test.go and privilege_cases_test.go); neither new acceptance-test file is reported

The public head remains the original GitHub-verified commit because GitHub reports the PR as mergeable and the current-main change does not overlap the patch.

license/cla is still pending and the CLA assistant reports it as not signed. I am therefore keeping the PR in Draft until the contributor completes that personal legal step; no CLA action or recheck was performed here.