Fix file persistence session initialization

[sueun-dev]

This fixes standalone file-backed connections failing during session metadata initialization when the memory catalog is not attached yet.

What changed:

• Attach an in-memory memory catalog on the missing-memory-catalog path before installing session metadata views.
• Restore standalone file-persistence sessions to the persisted physical catalog/search path after metadata setup.
• Add regression coverage for metadata initialization and a PG-wire reconnect path with file_persistence enabled.

Why:
Issue #561 reports file_persistence: true failing during connection startup with No catalog + schema named "memory" found. The metadata setup needs the compatibility views in memory, but standalone file-backed connections can start without that catalog attached.

Checked:

• go test -run TestInitSessionDatabaseMetadataWorksOnFileBackedDatabase ./server/sessionmeta
• go test -run TestFilePersistenceStandaloneConnectionInitializesSession ./server
• go test ./server/sessionmeta ./server
• just build
• just lint
• just test-unit
• Not run: Docker-backed integration tests because Docker is not installed in this local environment.

Fixes #561

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: c8a20ccfbe

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[chatgpt-codex-connector]

Guard file catalog selection on DataDir

When FilePersistence is true but DataDir is empty, DuckDBDSN/openBaseDB intentionally fall back to :memory: (covered by the existing empty-DataDir fallback test), so there is no catalog named after c.username. This new branch still runs USE <username> for that configuration and will turn every standalone connection into a FATAL failed to select file-backed catalog; gate this on an actual file-backed database, e.g. cfg.DataDir != "", so the in-memory fallback continues to work.

Useful? React with 👍 / 👎.