TEST: Stabilize Operations E2E

[Roman Lutz (romanlutz)]

Description

The seeded Operations test blocked the merge queue for #3315 in this run. The operation was registered correctly, but startup data moved the visible picker during its click. The dropdown stayed closed and never requested the operation list. Controlled browser diagnostics reproduced the same race when the runtime banner disappeared.

• Wait for runtime readiness and configured defaults before ordinary startup interactions, including both viewport variants of the proposed-name reload flow.
• Check that selected IDs and explicit operation: null values reach localStorage before navigation or reload. An empty picker alone is not proof of clearing: it is also empty while a saved-ID lookup is pending.
• Document the synchronization pattern. Leave intentional early-startup tests unchanged so they still exercise choices made before defaults arrive.

No product behavior, converter code, timeout increases, or fixed sleeps are introduced. Related PR #3275 adds rename/delete coverage and does not contain these fixes.

Tests and Documentation

Validated locally on Windows with the provider-free seeded backend and isolated ports. Both suites ran with CI=true, E2E_SEEDED_MODE=true, and zero retries.

Exact validation commands and results, run from frontend

uv run --no-sync node node_modules\playwright\cli.js test operations-acceptance.spec.ts operations.spec.ts --project seeded --workers 1 --retries 0 --fail-on-flaky-tests --trace on --reporter list --output 'C:\Users\romanlutz\.copilot\session-state\732186ad-b909-441f-93fb-93d18dac3c62\files\fixed-seeded-results'
# Passed: 9 tests.

uv run --no-sync node node_modules\playwright\cli.js test labels-operation-picker.spec.ts --project mock --workers 1 --retries 0 --fail-on-flaky-tests --reporter list --output 'C:\Users\romanlutz\.copilot\session-state\732186ad-b909-441f-93fb-93d18dac3c62\files\fixed-mock-results'
# Passed: 20 tests, including intentional early-startup choices.

node node_modules\eslint\bin\eslint.js e2e\_runtime.ts e2e\operations-acceptance.spec.ts e2e\operations.spec.ts --max-warnings 0
# Passed.

npm run type-check
# Passed.

node node_modules\typescript\bin\tsc --project 'C:\Users\romanlutz\.copilot\session-state\732186ad-b909-441f-93fb-93d18dac3c62\files\tsconfig.e2e-check.json' --noEmit
# Passed: explicit E2E type-check extending the repository configuration.

Session-only diagnostics also exercised the new helper with delayed defaults at both viewport widths and controlled runtime-banner reflow. All three checks passed. These diagnostic files and traces are not included in the PR.

Updated frontend/README.md with startup and preference synchronization guidance. No notebooks changed; JupyText and live-provider tests were not run.