fix(lambda-http): handle fallible response body errors

[IamPritamAcharya]

📬 Issue #, if available:

Closes #1165

✍️ Description of changes:

lambda_http accepts fallible HttpBody response bodies, but its buffered text
and binary conversion paths called expect() after body.collect().await.
Normal body errors therefore caused an internal panic.

This change:

• handles collection errors in both text and binary conversion paths;
• propagates failures from run, run_concurrent, runtime, and
  runtime_concurrent to lambda_runtime as invocation diagnostics;
• preserves the body error's concrete type and debug message in the diagnostic;
• returns a deterministic, header-free HTTP 500 with an empty body when
  IntoResponse is used directly;
• uses the same HTTP 500 fallback for the existing public Adapter, which must
  preserve its handler error type;
• keeps the public IntoResponse, ResponseFuture, Adapter, and
  TransformResponse signatures unchanged;
• preserves successful content-type, content-encoding, text, and binary
  conversion behavior;
• does not modify the separate response-streaming path discussed in [lambda_runtime] early client closure of a streaming response panics when using cargo lambda watch #1051.

Regression tests use a StreamBody that emits partial data and then returns an
UnexpectedEof. Both text and binary paths are covered, along with propagation
through the standard runtime adapter.

Validation performed:

• cargo +nightly fmt --all -- --check
• cargo +1.84.0 test --all-features --verbose --package lambda_http
• cargo check --workspace --all-features
• cargo test -p lambda_http --all-features
• cargo test --workspace --all-features --exclude lambda-integration-tests
• cargo clippy --workspace --all-features -- -D warnings
• RUSTDOCFLAGS='--cfg docsrs -Dwarnings' cargo +nightly doc --workspace --no-deps --document-private-items --all-features
• git diff --check

The deployment integration test requires an externally configured
TEST_ENDPOINT and was therefore not run successfully locally.

🔏 By submitting this pull request

• I confirm that I've ran cargo +nightly fmt.
• I ran Clippy with the repository CI flags; no automatic fixes were required.
• I confirm that I've made a best effort attempt to update all relevant documentation.
• I confirm that my contribution is made under the terms of the Apache 2.0 license.