feat(BA-6550): represent model service command as string

[seedspirit]

Summary

• Refactor model service start_command to use a single internal command string while preserving deprecated list input compatibility.
• Keep /bin/bash as the default shell, allow explicit null/empty shell for direct argv execution, and update kernel-side command resolution accordingly.
• Add an idempotent Alembic data migration for stored model definitions in revisions, revision presets, and runtime variant defaults.

Test plan

• pants fmt --changed-since=HEAD
• pants fix --changed-since=HEAD
• pants lint --changed-since=HEAD
• pants check --changed-since=HEAD
• pants test --changed-since=HEAD
• pants lint --changed-since=origin/main
• Push hook: lint/check/test on origin/main..HEAD@603d0918f
• Live manager/web API verification for explicit command, deprecated list start_command, vLLM default model definition, and revision preset storage

Resolves BA-6550

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📚 Documentation preview 📚: https://sorna--12445.org.readthedocs.build/en/12445/

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📚 Documentation preview 📚: https://sorna-ko--12445.org.readthedocs.build/ko/12445/

[Copilot]

Pull request overview

This PR (BA-6550) refactors the model service start_command from an argv list (list[str]) to a single internal command string, while keeping backward compatibility with deprecated list-form input and the legacy command/start_command API fields. Shell handling moves to the kernel: /bin/bash remains the default, but an explicit null/empty shell now triggers direct shlex.split argv execution. An idempotent Alembic data migration converts already-stored model definitions to the new string form across revisions, revision presets, and runtime variant defaults.

Changes:

• start_command is normalized to str | None internally; lists are folded via shlex.join, {model_path} substitution and arg-appending operate on strings, and the kernel splits/wraps based on shell.
• shell is widened to str | None (GQL/DTO/config), with explicit null preserved through to_resolved; output DTO exposes both new command and deprecated start_command.
• New data migration ba6615a4d2f1 stringifies stored argv lists; extensive test updates across manager/agent/kernel/common.

Reviewed changes

Copilot reviewed 21 out of 21 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/ai/backend/common/config.py	Core: start_command str, _resolve_start_command, string-based merge/append/placeholder
src/ai/backend/kernel/service.py	Shell None/empty handling: wrap vs shlex.split
src/ai/backend/agent/agent.py	Image-cmd fallback and legacy env args use shlex.join
src/ai/backend/common/dto/manager/v2/deployment/types.py	Output DTO command + deprecated start_command derivation
src/ai/backend/manager/api/adapters/deployment_revision_preset/adapter.py	Maps internal string to command + legacy list
.../alembic/versions/ba6615a4d2f1_*.py	Idempotent data migration; # Part of: placeholder unfilled
.../gql/deployment/types/revision_preset.py, request.py	shell widened to nullable
docs/.../*.graphql, changes/12445.enhance.md	Schema/changelog updates
tests/**	Updated expectations to string form; new shell/alias cases

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[fregataa]

This seems to be a feature job rather than an enhancement or refactoring since API schema changes.
And is this correct?

• API allows array typed start_command and shell is used only when start_command is set
• Agent and Kernel also allows start_command and shell is used only when start_command is set
• All Agent, Kernel and API uses command if it is set

[seedspirit]

@fregataa Yes, that's correct with a few details:

• The list-form start_command is kept for backward compatibility, but it is deprecated and will be removed in the future.
• Since both the deprecated start_command and the new command can currently be set, command takes precedence when both are provided.
• shell, start_command, and command are independent configuration fields.
• When shell is set, the command string is executed as shell -c command.
• When shell is null, the command string is split with shlex.split() and executed directly as argv.