fix: quote string literals with single quotes in Explain

[h2zi]

Symptom

Explain wraps string values in double quotes (name = "hello"), but double quotes are identifier quoting in SQLite — SQL copied from the logs misparses the value as a column reference (#197).

Fix

Use standard single-quoted string literals in Explain.

One coupled change is required: GORM embeds string default values into CREATE TABLE DDL through Dialector.Explain (FullDataTypeOf → DEFAULT 'hello'), and the DDL parser only stripped double quotes from parsed default values. With Explain fixed in isolation, DefaultValue() would return 'hello' with quotes and every AutoMigrate would try to re-alter the column. The parser now strips single quotes too, and keeps stripping double quotes so tables created by older driver versions still parse.

Tests

• TestExplainQuotesStrings — literal quoting.
• TestDefaultValueRoundTrip — default value parses back clean, a second AutoMigrate leaves the DDL untouched, and a legacy double-quoted default still parses.

Fixes #197

🤖 Generated with Claude Code

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Merge order (this batch: #241 → #242 → #243 → #244 → #245 → #246, plus #234 from the previous batch): all seven are functionally independent and merge cleanly in any order. Several append tests to the same test files, so whichever merges later may show a trivial append-only conflict in migrator_test.go/ddlmod_test.go — I'll rebase the remaining ones promptly after each merge, as before.

[Copilot]

Pull request overview

Updates the SQLite dialector’s SQL rendering to use correct SQLite string-literal quoting in Explain, and adjusts DDL default-value parsing to keep migrations idempotent when defaults are emitted via Explain.

Changes:

• Switch Dialector.Explain to use single-quoted string literals (avoids misparsing double quotes as identifiers in SQLite).
• Update parseDDL default-value extraction to strip string-literal quotes for both new (single-quoted) and legacy (double-quoted) tables.
• Add tests covering Explain string quoting and default-value round-tripping through AutoMigrate, including legacy double-quoted defaults.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
sqlite.go	Explain now emits single-quoted string literals for SQLite correctness.
ddlmod.go	Default-value parsing updated to remove surrounding quotes so migrations remain idempotent across versions.
sqlite_test.go	Adds coverage for Explain quoting and default parsing / idempotent AutoMigrate.

Suppressed comments (1)

sqlite_test.go:184

• The result of db.Raw(...).Scan(&after) is not checked. If the query fails, after stays empty and the test may pass/fail for the wrong reason. Capture and assert the error from Scan.

	var after string
	db.Raw("SELECT sql FROM sqlite_master WHERE type='table' AND name='explain_defaults'").Scan(&after)

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