fix(BA-6062): Wrap legacy string based start_command via shell -c instead of shlex.split

[seedspirit]

Summary

• Replace shlex.split normalization with shell -c wrapping for legacy string start_command in stored model definitions, preserving shell semantics (pipes, redirections, env expansion, multiline). Effective on data-hydration paths only — see Scope below.
• Add Alembic migration 70ffcaaa5f5c that re-wraps existing single-element argv lists whose only token contains whitespace. Repairs rows produced by prior migration 8c1f7d3a9e2b which stored legacy shell strings as a meaningless one-token list like ["python service.py"].
• Helper _wrap_str_start_command_into_argv is non-mutating: returns a new dict with overridden start_command only when wrapping is needed.

Scope

The wrap fires at the strict-type boundary (ModelServiceConfig.model_validate) and on draft hydration of stored payloads:

• DB read via PydanticColumn(ModelDefinition) / PydanticColumn(ModelDefinitionDraft).
• vfolder model-definition.yaml validation through model_definition_iv (trafaret).
• Preset / runtime variant JSONB columns hydrated into ModelDefinitionDraft.

REST v2 / GraphQL request input is not affected: ModelServiceConfigInput (common/dto/manager/v2/deployment/request.py) still types start_command as list[str] | None, so request payloads must be argv lists. The wrap is purely a backward-compat for pre-existing stored data and user-authored yaml.

Test plan

• Alembic upgrade/downgrade/re-upgrade verified locally on 5 representative rows in runtime_variants (whitespace 1-elem → rewrap, no-ws 1-elem preserved, multi-elem preserved, null preserved, custom-shell vs DEFAULT_SHELL fallback). Downgrade no-op, re-upgrade idempotent.
• Unit: tests/unit/manager/data/deployment/test_model_definition_start_command_compat.py covers Pydantic, trafaret, raw YAML, and PydanticColumn read paths with both default-shell and custom-shell cases.
• CI: pants test --changed-since=... green.

Resolves BA-6062

[Copilot]

Pull request overview

This PR changes legacy string start_command handling for model service definitions from shlex.split() normalization to shell execution via [shell, "-c", command], plus a migration intended to repair previously persisted malformed single-element argv values.

Changes:

• Adds shared default shell wrapping for string start_command in Pydantic and trafaret validation paths.
• Adds Alembic migration 70ffcaaa5f5c to rewrite selected persisted single-element argv lists.
• Updates compatibility tests and adds a changelog entry.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 6 comments.

File	Description
src/ai/backend/common/config.py	Replaces shlex.split coercion with shell wrapping and updates default shell usage.
src/ai/backend/manager/models/alembic/versions/70ffcaaa5f5c_rewrap_legacy_string_start_command_via_.py	Adds data migration for previously wrapped single-element start_command lists.
tests/unit/manager/data/deployment/test_model_definition_start_command_compat.py	Updates tests to expect [shell, "-c", command] wrapping.
changes/11648.fix.md	Adds release note for the compatibility fix and migration.

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