bedrock: Fix remaining streaming delays (#33931)

Closes #26030

*Note: This is my first contribution to Zed*

This addresses a second streaming bottleneck in Bedrock that remained
after the initial fix in #28281 (released in preview 194).

The issue is in the mechanism used to convert Zed's internal `AsyncBody`
into the `SdkBody` expected by the Bedrock language provider. We are
using a non-streaming converter that buffers responses.

**How the fix works:**
The AWS SDK provides streaming-compatible converters to create `SdkBody`
instances, but these require the input body to implement the `Body`
trait from the `http-body` crate.

This PR enables streaming by implementing the required trait and
switching to the streaming-compatible converter.

**Changes (2 commits):**

 * 1st Commit - **Implement http-body Body trait for AsyncBody:**
   - Add `http-body = 1.0` dependency (already an indirect dependency)
   - Implement the `Body` trait for our existing `AsyncBody` type
- Uses `poll_frame` to read data chunks asynchronously, preserving
streaming behavior

 * 2nd Commit - **Use streaming-compatible AWS SDK converter:**
- Create `SdkBody` using `SdkBody::from_body_1_x()` with the new `Body`
trait implementation

**Details/FAQ:**

**Q: Why add another dependency?**
A: We tried to avoid adding a dependency, but the AWS SDK requires the
`Body` trait and `http-body` is where it's defined. The crate is already
an indirect dependency, making this a reasonable solution.

**Q: Why modify the shared `http_client` crate instead of just
`aws_bedrock_client`?**
A: We considered implementing the `Body` trait on a wrapper in
`aws_bedrock_client`, but since `AsyncBody` already uses `http` crate
types, extending support to the companion `http-body` crate seems
reasonable and may benefit other integrations.

**Q: How was this bottleneck discovered?**
A: After @5herlocked's initial streaming fix in #28281, I tested preview
194 and noticed streaming still had issues. I found a way to reproduce
the problem and chatted with @5herlocked about it. He immediately
pinpointed the exact location where the issue was occurring, his
diagnosis made this fix possible.

**Q: How does this relate to the previous fix?**
A: #28281 fixed buffering issues higher in the stack, but unfortunately
there was another bottleneck lower-down in the aws-http-client. This PR
addresses that separate buffering issue.

**Q: Does this use zero-copy or one-copy?**
A: The `Body` implementation includes one copy. Someone more
knowledgeable might be able to achieve a zero-copy approach, but we
opted for a conservative approach. The performance impact should not be
perceptible in typical usage.

**Testing:**
Confirmed that Bedrock streaming now works without buffering delays in a
local build.


Release Notes:

- Improved Bedrock streaming by eliminating response buffering delays

---------

Co-authored-by: Marshall Bowers <git@maxdeviant.com>

Cargo.toml

@@ -482,6 +482,7 @@ heed = { version = "0.21.0", features = ["read-txn-no-tls"] }
 hex = "0.4.3"
 html5ever = "0.27.0"
 http = "1.1"
+http-body = "1.0"
 hyper = "0.14"
 ignore = "0.4.22"
 image = "0.25.1"