fix(ci): drop the blocked third-party Rust setup action

[PaulAsjes]

The bug

No CI has ever run in this repo. Every ci.yml run — on next, on the regeneration PRs, on every feature branch — ends in startup_failure:

$ gh run list --workflow=ci.yml
feat/tests-group                    pull_request  startup_failure
next                                push          startup_failure
fern-bot/2026-07-28_15-36-43_370    push          startup_failure
...

Cause

The org restricts GitHub Actions to an allowlist:

{ "allowed_actions": "selected", "github_owned_allowed": true, "verified_allowed": false,
  "patterns_allowed": ["elevenlabs/actions/*", "pnpm/action-setup@*", ...] }

actions-rust-lang/setup-rust-toolchain isn't on it. Referencing a disallowed action fails the entire workflow before any job starts — which is exactly why release.yml works (it only uses GitHub-owned actions/*) while ci.yml never has.

Fix

The GitHub-hosted ubuntu runners already ship a stable Rust toolchain, so the setup action isn't needed. Removed it and use cargo directly, printing rustc/cargo --version for debuggability. Added a comment so nobody reintroduces a blocked action.

This PR is self-verifying: for same-repo PRs the pull_request event runs the workflow from the PR branch, so if CI goes green here, the fix works.

Why this matters now

PR #112 added a pull_request trigger and steps to run the generated crates' 146 previously-unrun tests. That trigger fires correctly — but the workflow still couldn't start. With this fix, all of it actually executes: 1913 tests across the framework, workflow, and both generated crates.

Possible follow-up

The three jobs each rebuild from scratch. actions/cache is GitHub-owned (so allowlisted) and could cache ~/.cargo + target/ — happy to add if the runtime is annoying.

🤖 Generated with Claude Code