test(core): use disposable session fixtures

[kitlangton]

Why

Three Session test boundaries manually reproduce the async-disposal protocol already implemented by Effect.acquireDisposable. The prompt image case additionally bypasses the shared temporary-directory fixture, losing its canonical path and busy-directory cleanup behavior.

What Changes

• Use Effect.acquireDisposable for the two session-create acquisitions and the replay acquisition in session-message-update.
• Build the prompt image fixture with the existing fixture/tmpdir helper instead of direct mkdtemp/rm.
• Keep every existing test scope, callback lifetime, target layer, assertion, and fixture cleanup error visible.

Scope

This completes the still-unowned TESTS-03 occurrences. #45637 owns the same cleanup in move, remove, revert, and view tests, with no file overlap. #45680 touches the active-session assertion in session-prompt.test.ts, not this image fixture.

Open same-file PRs have no behavioral dependency: #45735 adds a session-create case that calls the existing withTmp boundary; #45336 adds a later session-message-update case; neither changes these acquisitions. #45575 changes other prompt fixtures and its import hunk adds new mkdtemp users, so whichever PR lands second must reconcile that import block while preserving both independent fixture changes.

Verification

# packages/core
bun run test test/session-create.test.ts test/session-message-update.test.ts test/session-prompt.test.ts
bun typecheck

# repository root
bunx prettier --check packages/core/test/session-create.test.ts packages/core/test/session-message-update.test.ts packages/core/test/session-prompt.test.ts
bunx oxlint packages/core/test/session-create.test.ts packages/core/test/session-message-update.test.ts packages/core/test/session-prompt.test.ts
bunx ast-grep scan -c script/ast-grep/effect-simplifications/sgconfig.yml --off=unused-suppression packages/core/test/session-create.test.ts packages/core/test/session-message-update.test.ts packages/core/test/session-prompt.test.ts
git diff --check
git push -u origin disposable-session-tests

The focused suites passed 90 tests with 271 assertions. Core typecheck, structural scan, whitespace checks, and the normal pre-push 33-package workspace typecheck passed. Oxlint reported no errors and six existing warnings. Prettier accepts every changed line but its full-file check still flags an unrelated pre-existing multiline expression in session-prompt.test.ts; that formatter-only change was deliberately excluded from this PR.