Improvement: DO-12330 speed up E2E CI

[krzysztof-causalens]

Motivation and Context

The E2E job is currently the slowest CI job after the Oxlint/Oxfmt tooling upgrade. It also sits behind a shared setup job, so every real test job waits for setup even when its caches are already warm.

Measured GitHub Actions results from master run 30278046973 and this PR's run 30283988882:

Measurement	master	This PR
Workflow wall time	7m 38s	4m 36s
Shared setup job	1m 42s	removed
E2E job	5m 42s	4m 27s
Cypress test step	3m 13s	2m 40s

The E2E job is 1m 15s faster (22%), while the workflow critical path is 3m 02s faster (40%). GitHub-hosted runner timings naturally include some run-to-run noise.

Implementation Description

• Upgrade Cypress from 10.11.0 to 15.19.0 and fix the stricter subject handling introduced by newer Cypress releases.
• Cache the default authentication session across specs with cy.session, while continuing to visit each test route independently.
• Remove the shared setup job so lint, unit-test, and E2E jobs can begin immediately.
• Preserve cold-cache correctness by installing the Python environment and Cypress binary in the consuming job on cache misses.
• Let actions/setup-node manage the pnpm store cache and remove the redundant apt update/wget installation.
• Rely on the GitHub-hosted Ubuntu image's Cypress runtime libraries instead of downloading, installing, and caching an outdated APT dependency set on every new cache key.
• Use shallow checkouts in test jobs and let cypress run validate the restored binary instead of launching Cypress once in a separate verification step.

Controlled local measurements, included only to separate the Cypress and authentication effects:

Configuration	Cypress-reported time	Local wall time
Cypress 10 + cached authentication	83s	100.7s
Cypress 15 + cached authentication	75s	98.2s
Cypress 15 + original authentication flow	84s	104.1s

Any new dependencies Introduced

No new dependency category. The existing Cypress development dependency is upgraded from 10.11.0 to 15.19.0.

How Has This Been Tested?

• Ran the full 55-test E2E suite successfully on both Cypress 10.11.0 and 15.19.0.
• Re-ran the navigation compatibility spec on the final Cypress 15 state.
• Checked the affected navigation manually in a local browser.
• Ran make lint-js.
• Ran pnpm lerna run format:check.
• Verified the frozen pnpm lockfile, Cypress binary, workflow YAML, and git diff --check.
• Passed the complete GitHub Actions workflow without the explicit Cypress APT dependency step.
• Passed follow-up run 30285912316 using depth-1 checkouts and direct Cypress launch from the restored binary cache.

PR Checklist:

• I have implemented all requirements? (see JIRA, project documentation).
• I am not affecting someone else's work, If I am, they are included as a reviewer.
• I have added relevant tests (unit, integration or regression).
• I have added comments to all the bits that are hard to follow.
• I have added/updated Documentation.
• I have updated the appropriate changelog with a line for my changes. (Not applicable: internal CI/test tooling only.)

Screenshots (if appropriate):

Not applicable; this changes test and CI infrastructure only.

[sam-causalens]

LGTM