fix(server): fall back to the connected tab when a lease cannot launch

[kushals256]

Summary

• reticle_lease dead-ended when Chromium could not launch (missing binary, acquire throw), even when a human tab was already connected and driveable — including one flagged throttled. Reporters then drove that tab by hand and it worked.
• When launch fails and reticle_sessions already lists a live non-leased tab, acquire now returns that session with fellBackToExisting: true and expiresInMs: 0. It is not a pool lease; the HUD stays on that tab. Isolation still mints when launch works.
• Windows Playwright resolution and the doctor mismatch from Lease Chromium failures all report the same wrong cause, and there is no fallback #691 are untouched.

Addresses the fallback ask of #691.

Test plan

• Chromium-absent preflight with a live tab returns that session and never calls the pool
• acquire throw with a live tab returns that session instead of "is the app running?"
• Chromium-absent with no tab still throws the install hint
• Existing "never refuses the lease" path still mints when launch works
• pnpm --filter @reticlehq/server exec tsc -b and lease-tools unit tests green

Made with Cursor

[divshekhar]

Reviewed. The fallback is right and I want it in — one change requested first, from a duplicate PR that arrived 11 hours after yours.

#871 (@Bhumika-1432006, now closed in your favour) implemented the same ask with a different envelope, and theirs is better on one specific point:

// yours
{ "fellBackToExisting": true, "expiresInMs": 0 }
// theirs
{ "noManagedBrowser": { "sessionId": "...", "reason": "Chromium is not installed …" } }

Please carry the reason. fellBackToExisting: true tells the agent it did not get isolation; it does not tell it why, and the three causes have three different fixes — a missing binary wants install chromium, a missing shared library wants install-deps, and a build mismatch wants neither. chromiumHint(probe) already composes exactly that sentence, so this should be threading a string you already have rather than new logic.

Two smaller notes, both optional:

• expiresInMs: 0 as the "this is not a real lease" signal works, but it is a sentinel a caller has to know to read. A named field says it outright. Your call — fellBackToExisting already carries the meaning, so this is taste rather than correctness.
• Their description puts the severity better than ours does, and it is worth stealing for the changelog entry: a lease is what the daemon actively recommends when a tab is hidden or throttled, so refusing at exactly that moment strands the agent with no path forward while a driveable session sits right there.

Everything else — covering the preflight and the launch, keeping isolation when launch works, and the throttled-tab case — is exactly what #691 asked for. Add the reason and I will merge it.