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

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