[ENHANCEMENT] Improve table migration: handle column overrides and fix Value column naming

[ibakshay]

Description

Improves the Grafana-to-Perses table migration CUE script to handle previously-unsupported field override properties and fixes the Value #X → value #N column naming mismatch.

Fixes perses/perses#4063

Problem

When migrating a Grafana table panel with multiple queries and column overrides, several issues occurred:

1. Column naming mismatch — Grafana names multi-query value columns by refId (Value #A, Value #B), but Perses uses 1-based query index (value #1, value #2). The migrated column settings didn't match actual rendered column names, making them ineffective.
2. Missing column overrides — Grafana field overrides for unit, mappings, and noValue were completely ignored during migration, resulting in:
   • No unit formatting on columns (e.g., percent)
   • No value mappings (e.g., 0 → "False", 1 → "True")
   • No fallback text for null values

Changes

1. Fix Value column naming (refId → index)

Builds a mapping from the targets array to convert Grafana's refId-based naming to Perses's index-based naming automatically:

• Value #A → value #1 (first query)
• Value #B → value #2 (second query)
• ....

2. Handle unit override → per-column format

Grafana field overrides with "id": "unit" are now converted to format: {unit: "..."} on the corresponding column setting. Only units recognized by Perses are emitted (e.g., percent, bytes, seconds). Unrecognized Grafana-specific units (e.g., decgbytes, dectbytes) are skipped.

3. Handle mappings override → per-column cellSettings

Grafana field overrides with "id": "mappings" (value-type) are now converted to column-level cellSettings entries with Value conditions.

4. Handle noValue override → per-column cellSettings

Grafana field overrides with "id": "noValue" are now converted to a cellSettings entry with a Misc condition for "null", displaying the specified fallback text when a cell has no value.

I have added couple of tests for these changes. All the existing 23 tests are passing as well.

Limitations

• Grafana units without a Perses equivalent (decgbytes, dectbytes, kbytes, mbytes, gbytes, tbytes) are not migrated. These columns are left unformatted. A future Perses enhancement could add support for these scaled byte units.
• The JoinByColumnValue transform cannot be inferred automatically from Grafana's merge transformation (which works implicitly). Users may need to add this manually after migration for multi-query tables.

Checklist

• Pull request has a descriptive title and context useful to a reviewer.
• Pull request title follows the [<catalog_entry>] <commit message> naming convention using one of the
  following catalog_entry values: FEATURE, ENHANCEMENT, BUGFIX, BREAKINGCHANGE, DOC,IGNORE.
• All commits have DCO signoffs.

UI Changes

• Changes that impact the UI include screenshots and/or screencasts of the relevant changes.
• Code follows the UI guidelines.

[Nexucis]

awesome, thank you @ibakshay !