[ZEPPELIN-6666] Add notebook transport fixtures

[voidmatcha]

What is this PR for?

Adds a versioned Notebook REST/WebSocket transport-fixture foundation for the Shared Notebook Core migration.

A fixture is a boundary contract, not a UI mock: it records observable REST request/response shapes, WebSocket frames, and their ordering. The strict replay adapter then fails on unexpected, reordered, or unconsumed traffic, preventing Angular and future React adapters from silently drifting.

Captured fixtures require a scenario, owner, covered operations, and known exclusions. Sensitive values are redacted before a fixture is written. Binary WebSocket capture is rejected until a redaction policy exists.

The fixture-only suite and the live-capture suite are separate. The live suite runs against an isolated, authenticated Zeppelin instance and captures traffic from a real notebook page.

This follows the contract-suite direction used by Apache Hadoop:

• https://github.com/apache/hadoop/blob/trunk/hadoop-common-project/hadoop-common/src/site/markdown/filesystem/testing.md
• https://github.com/apache/hadoop/blob/trunk/hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/fs/contract/ContractOptions.java

What type of PR is it?

Improvement

What is the Jira issue?

ZEPPELIN-6666

How should this be tested?

• npm run check:core-contract-fixtures — 32 passed
• CI=true npm run e2e:core-contract — 40 passed
• CI=true PLAYWRIGHT_BASE_URL=http://127.0.0.1:18080 npm run e2e:core-contract:live -- --project=chromium --retries=0 — 1 live capture passed against an isolated authenticated Zeppelin server

Screenshots (if appropriate)

N/A

Questions:

• Does the license files need to update? No
• Is there breaking changes for older versions? No
• Does this needs documentation? Yes, e2e/core-contract/README.md