Adjust warnings, import errors, and Dag source models for sdk/importers

[dilnazanlid]

• Project Board: AIP-85 Dag Importer (Project #672)
• Related PR: [AIP-85] Add Dag importers to Task SDK in preparation for rewiring #72369 ([AIP-85] Move DAG importer abstractions and registry into Task SDK)

Why

AIP-85 allows Dags to come from non-Python and composite sources (YAML/JSON importers,
ZipImporter, external bundles). Three pieces of metadata currently assume a single local .py file:

1. Import errors only track filename, but a Dag definition is no longer strictly 1:1 with a file —
   an archive member inside a container (e.g. archive.zip/dags/my_dag.py) needs an abstract source
   reference while retaining filename for compatibility.
2. DagWarning rejects any type outside the built-in enum, preventing importers from reporting
   format-specific warnings.
3. Dag source is always highlighted as Python in the UI, regardless of the Dag's actual definition language.

This is groundwork for AIP-85: it introduces the necessary schema and model extensions without altering
runtime behavior for existing Python Dags.

What

Import errors: add source_reference, keep filename

• Migration ca8499dc1004 adds a nullable source_reference column (String(2000)) to import_error.
• Retains filename as the primary file path column across the database, model, and API to preserve
  full backward compatibility for existing tooling and API consumers.
• Exposes source_reference: str | None on ImportErrorResponse and ParseImportError.
• Implements ParseImportError.full_file_path() using Path.is_absolute() rather than substring prefix
  matching, avoiding false positives across sibling bundle paths.
• Streamlines delete_dag cleanup to delete import errors matching source_reference or filename.

Dag warnings: open-ended types, enum retained

• DagWarning.__init__ accepts DagWarningType | str, normalizing enum members to their string value
  and preserving importer-supplied warning types verbatim.
• DAGWarningResponse.warning_type and the list_dag_warnings filter accept DagWarningType | str.
  This retains DagWarningType in the published schema and generated clients as the vocabulary for
  core-emitted warnings, while gracefully accepting importer-defined warnings.
• Filtering by an unknown warning_type returns 200 with an empty list instead of 422.

Dag source language

• Migration e5a91c7f42b3 adds a nullable language column (String(64)) to dag_code.
• DAGSourceResponse and DagCode persist and expose language: str | None.
• UI Code view passes code?.language ?? "python" to Monaco Editor and CodeDiffViewer, defaulting
  to Python when unstated.

Was generative AI tooling used to co-author this PR?

• Yes

Generated-by: Google Gemini following the guidelines

[jason810496]

Thanks for the update.

[jason810496]

Just head up that I will take over this PR and get this merge this week since GCP team is OOO this week.
Dilnaz continue on the work.

[shahar1]

Great work!

[pierrejeambrun]

LGTM