[Streams] Unskip MKI Tech Preview badge test

[rStelmach]

closes: #258589

Summary

Re-enables the Streams UI test should not show Technical Preview badge when AI suggestions are unavailable on serverless observability (MKI). The test was skipped in #263984 while it was failing deterministically on the MKI.
The underlying bug was fixed by #263843 after this test was skipped

Root cause

Root cause of the original failure (fixed):
Before #263843, useLoadConnectors in @kbn/inference-connectors had a client-side fallback that called fetchConnectorById against GET /internal/inference/connectors/{id} whenever soEntryFound === false and defaultConnectorId was truthy. The test's page.route mock only covered **/internal/search_inference_endpoints/connectors*, so the fallback slipped past the mock and returned the project's EIS connector. The Technical Preview badge (and the "Elastic Managed LLM is the new default" callout) then rendered, and the assertion failed.

This was MKI-only because the fallback is gated on defaultConnectorId being truthy, which requires the GEN_AI_SETTINGS_DEFAULT_AI_CONNECTOR uiSetting to be configured. Serverless observability CI projects preconfigure that setting to an EIS inference endpoint; local stateful / ECH environments do not, so the fallback short-circuited there.

How #263843 fixed it:
Removed the client-side fallback entirely. Default-connector merging now happens server-side in mergeConnectors (x-pack/platform/plugins/shared/search_inference_endpoints/server/lib/merge_connectors.ts). After the fix, useLoadConnectors makes exactly one HTTP call to the endpoint the test already mocks.

[macroscopeapp]

Catch flakiness early (recommended)

Recommended before merge: run the flaky test runner against this PR to catch flakiness early.

Trigger a run with the Flaky Test Runner UI or post this comment on the PR:

/flaky scoutConfig:x-pack/platform/plugins/shared/streams_app/test/scout/ui/playwright.config.ts:30

This check is experimental. Share your feedback in the #appex-qa channel.

^{Posted via Macroscope — Flaky Test Runner nudge}

[elasticmachine]

Pinging @elastic/obs-onboarding-team (Team:obs-onboarding)

[macroscopeapp]

Approvability

Verdict: Would Approve

This PR unskips a test for MKI environments by updating test tags from a local-only string to the standard serverless tags. The change is limited to test configuration with no production runtime impact.

^{Macroscope would have approved this PR. A repo admin can enable approvability here.}