fix: handle non-string title in export dashboard standardization

[mrodm]

Summary

elastic-package export dashboards panicked when processing Kibana dashboards
that contain XY chart panels. The root cause was an unsafe type assertion in
standardizeObjectProperties: the code assumed the title key was always a
string, but in axis config objects (embeddableConfig.axis.x/y/y2) it is a
nested map like {"visible": false}.

Example (snippet from dashboard export):

{
  "x": {
    "domain": {
      "rounding": false,
      "type": "fit"
    },
    "grid": {
      "visible": true
    },
    "labels": {
      "orientation": "horizontal"
    },
    "scale": "temporal",
    "ticks": {
      "visible": true
    },
    "title": {
      "visible": false
    }
  },
  "y": {
    "domain": {
      "rounding": true,
      "type": "full"
    },
    "grid": {
      "visible": true
    },
    "labels": {
      "orientation": "horizontal"
    },
    "scale": "linear",
    "ticks": {
      "visible": true
    },
    "title": {
      "visible": false
    }
  }
}

Changes

• internal/export/transform_standardize.go: Replace the direct type
  assertion value.(string) for the title key with a safe assertion. When
  title is not a string, the transformation is skipped and the value falls
  through to the general map/array recursion unchanged.
• internal/export/transform_standardize_test.go (new): Unit tests for
  standardizeObjectProperties covering:
  • String title with/without ECS suffix
  • Map-typed title (the axis config case) — no panic, value preserved
  • Nested map and array traversal
  • Markdown link rewriting via adjustObjectID

Tested with dashboards defined in elastic/integrations#20265

Proposed commit

fix: handle non-string title values in standardizeObjectProperties

In Kibana dashboard JSON, axis config objects (embeddableConfig.axis.x/y/y2)
use "title" as a nested map (e.g. {"visible": false}) rather than a string.
The previous direct type assertion panicked in those cases. Switch to a safe
assertion so non-string title values fall through to the general map/array
recursion unchanged.

Add unit tests covering: string title with/without ECS suffix, map-typed
title (axis case), nested and array traversal, and markdown link ID
adjustment.

---

This PR was generated with the assistance of Claude (claude-sonnet-4-6).

[teresaromero]

🐛 thanks!

[mrodm]

Tested again with the changes in this PR elastic/integrations#20265
before they were updated to 9.4.3 (using changeset elastic/integrations@8d6cd21)