[HDX-1360] External API parity for number-tile color

[alex-fedotyev]

Follow-up to #2265 (number-tile static color picker).

PR #2265 adds an optional color field to the internal SharedChartSettingsSchema. The external dashboards API (packages/api/src/utils/zod.ts) does not yet accept the field, so customers can only author colors through the UI.

Scope

Mirror the color field on the external API for number-tile chart configs and verify round-trip parity.

• packages/api/src/utils/zod.ts: add color: chartPaletteTokenSchema.optional() to externalDashboardNumberChartConfigSchema (and introduce the shared chartPaletteTokenSchema alongside the existing schemas).
• packages/api/src/routers/external-api/v2/utils/dashboards.ts: round-trip the field in both convertToExternalTileChartConfig and convertToInternalTileConfig (number-tile arm only).
• packages/api/src/routers/external-api/v2/dashboards.ts: OpenAPI JSDoc components for the palette enum.
• packages/api/openapi.json: regenerate via yarn workspace @hyperdx/api docgen.
• packages/api/src/routers/external-api/__tests__/dashboards.test.ts: extend the existing round-trip tests with color on a number tile.

Acceptance

• POST /api/v2/dashboards accepts a number-tile config with color: "chart-success".
• GET returns the same token unchanged.
• Invalid token (e.g. "chart-99", hex) is rejected with a clear schema error.
• OpenAPI spec lists the palette enum on the number-tile schema.

Relates to #1360.