feat(backend): Add plugin input/output fields to RecurringRun and Run protobufs

[hbelmiro]

Description of your changes:
Adds a generic plugin metadata interface to Run and RecurringRun as specified in KEP-12862. This is the schema-only change -- no MLflow or plugin business logic is introduced.

• Add MetadataValue, PluginOutput messages and plugins_input (field 19) / plugins_output (field 20) to Run
• Add plugins_input (field 19) to RecurringRun
• Propagate through Go models (*LargeText), storage layer (SQL NULL for absent), and API converters
• Regenerate Go gRPC, swagger, and HTTP client code

Details

• Model fields use *LargeText with nil = absent (SQL NULL), never empty string
• Storage write paths use largeTextToNullableSQL; read paths use pointer assignment
• GetRun/ListRuns/GetRecurringRun return plugins fields as stored, no external calls
• Existing function signatures (toApiRun, toApiRecurringRun, etc.) are unchanged

Test plan

• Unit tests for JSON serialization helpers (nil, empty, valid, malformed)
• Converter tests for toModelRun, toApiRun, toModelJob, toApiRecurringRun (with data + nil)
• Storage round-trip tests (create, get, update, list) for both runs and jobs
• DB-level NULL assertions verifying absent fields produce SQL NULL, not empty string
• Proto golden file tests updated

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[google-oss-prow]

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[hbelmiro]

/retest

[nsingla]

Don't we need number, text types here?

[hbelmiro]

ContentType is a UI rendering hint, not a data type. The value itself is google.protobuf.Value which already carries type information (string, number, etc.). UNSPECIFIED means "render as-is" which covers plain text and numbers. URL is the only case that needs special rendering (hyperlink).
Maybe we should renamen it from ContentType to UIHint or something to avoid confusing. But that should be done in the KEP first.

[nsingla]

This might be my lack of knowledge, why are we adding pluginsinput to Job?

[hbelmiro]

Job is the Go model for RecurringRun.

[hbelmiro]

/retest

[VaniHaripriya]

@hbelmiro Is AutoMigrate sufficient for adding plugins_input and plugins_output to run_details table, or should we include an explicit DB migration script to avoid schema discrepancies ?