Upgrade to ACP SDK 2.1 and futures 0.3.34. Remove the dedicated-thread
polling workaround introduced in #62259 now that the SDK bounds
dispatch-chain stack usage.

This is the first PR from the ACP integration work. First-class
tool-call names, context compaction support, and the independent native
elicitation ID fix are intentionally deferred to separate PRs.

- Upgrade the ACP SDK and derive crate to 2.1.0, with schema 1.7.0.
- Upgrade the nine futures-rs 0.3 components to 0.3.34, including the
[cloned-waker identity
fix](https://github.com/rust-lang/futures-rs/pull/3032) identified
through [ACP
rust-sdk#254](https://github.com/agentclientprotocol/rust-sdk/issues/254).
- Poll ACP connections on the normal GPUI background executor and remove
the now-unused ACP-specific dedicated-spawn wrapper. Keep the general
scheduler dedicated-executor APIs intact.
- Use the stable authentication metadata accessor and route first-class
terminal authentication without requiring ACP beta. Keep legacy
terminal-auth metadata as a fallback for other authentication methods.

Also upgrades a big fix release of the futures crates because there was
some polling issues with FuturesUnordered reported with the old
versions.

Release Notes:

- Improved ACP compatibility and async task wakeup handling.