Add release-validation test coverage for post-v0.8.7 merged PRs #951

Problem

The 8 PRs merged since v0.8.7 (#844, #937, #933, #893, #941, #930, #940, #929) added meaningful new behavior — non-interactive question handling, IDE-aware MCP/datamate transport + enabled-state persistence, a session-transcript REST endpoint, the Windows PowerShell installer, headless serve self-upgrade, and dbt error-bubbling — but several new code paths (error branches, env-var/config permutations, transport selection, installer safety) lack dedicated regression tests.

Ahead of the next release we want explicit coverage that locks in the intended behavior of these changes and documents the edge cases found during review.

Proposed work

Add release-validation regression tests covering each merged PR's new behavior and error paths, including static-analysis assertions for install.ps1 (PowerShell can't run in CI). Mark any genuine, pre-existing edge bugs as test.todo so they're tracked without blocking.

Acceptance

• New tests pass and are typecheck-clean
• No source files modified
• Edge cases surfaced during review are captured (as assertions or documented test.todo)