Remove the now-dead useFirebaseToken plumbing in useSessionCookie

[christian-byrne]

Residual from the review of #14613. Non-blocking, cleanup.

After #14613, src/platform/auth/session/useSessionCookie.ts:85 reads:

const usesFirebaseToken = isCloud || useFirebaseToken

All three public entry points (ensureSessionCookie at :136, createSession at :141, createSessionOrThrow at :150) open with if (!isCloud) return, so establishSession is unreachable off Cloud. That makes usesFirebaseToken unconditionally true on every reachable path, which in turn makes the single-flight join condition inert:

if (
  inFlightCreateSession?.ownerUid === ownerUid &&
  (!usesFirebaseToken || inFlightCreateSession.usesFirebaseToken)
) {

Both disjuncts are constant true, so the condition reduces to the ownerUid match. The useFirebaseToken parameter threaded through establishSession/performCreateSession/getSessionHeaderOrThrow, the usesFirebaseToken field on InFlightCreateSession at :7, and the authStore.getAuthHeader() fallback at :54-58 are all now vestigial.

Consequence: the assertion expect(mockGetAuthHeader).not.toHaveBeenCalled() in useSessionCookie.test.ts:221 tests a branch no build can execute, so it reads as coverage while providing none.

Either delete the dead plumbing, or, if the weak-credential path is meant to come back, restore a reachable caller for it. Note the ADR-0011 invariant 4 property the original test protected (a strict creation must never inherit a weaker request's credential) is currently unenforceable either way.

Thread: #14613 (comment)

[coderabbitai]

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#14290 - fix: isolate and preserve workflow drafts on workspace switch [merged]
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[christian-byrne]

Fixed and merged. Zero remaining references to useFirebaseToken or usesFirebaseToken in src/platform/auth/session/useSessionCookie.ts on main: the parameter, the InFlightCreateSession field, the dead authStore.getAuthHeader() fallback and the inert half of the single-flight join are all gone, and the join now reduces to the ownerUid match. Landed via #14613.