Refactor HTTP client request handling

[heaths]

Add FileStream API for easier streaming of request bodies e.g., from a file including a FileReader for tokio::fs::File. Makes some methods like SeekableStream::len() async since underlying APIs for file access are async.

Fixes #3971

[Copilot]

Pull request overview

This PR refactors async body/stream handling across core HTTP and storage layers to better support seekable streaming uploads (e.g., file-backed streams), and updates call sites/tests/examples accordingly.

Changes:

• Made SeekableStream::len/is_empty and Body::len/is_empty async and updated dependent policies, clients, and tests.
• Added buffered FileStream<T> plus a Tokio FileReader adapter to enable streaming file uploads without loading entire files into memory.
• Updated Azure Storage blob partitioned upload pipeline to work with async length, and added an end-to-end upload example using streaming.

Reviewed changes

Copilot reviewed 30 out of 31 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
sdk/storage/azure_storage_blob_test/src/lib.rs	Updates test helpers to use async len() for seekable bodies.
sdk/storage/azure_storage_blob/src/streams/partitioned_stream.rs	Makes constructor async and caches total length for partition buffering.
sdk/storage/azure_storage_blob/src/partitioned_transfer/upload.rs	Updates partitioned upload flow to use async body length and async PartitionedStream::new.
sdk/storage/azure_storage_blob/src/clients/block_blob_client.rs	Updates content-length computations to await async len().
sdk/storage/azure_storage_blob/examples/blob_storage_upload_file.rs	New example showing in-memory vs streamed file upload (Tokio + FileStream).
sdk/storage/azure_storage_blob/Cargo.toml	Adds example dependencies/features (e.g., clap, tokio/fs).
sdk/keyvault/azure_security_keyvault_keys/src/authorizer.rs	Adapts Key Vault auth flow to async Body::is_empty/len.
sdk/identity/azure_identity/src/managed_identity_credential.rs	Test adjustment for header iteration typing.
sdk/cosmos/azure_data_cosmos/src/fault_injection/http_client.rs	Minor refactor of request cloning/forwarding.
sdk/core/typespec_macros/tests/data/safe-debug-tests/tests/safe-debug.rs	Gates a test to avoid running under the debug feature.
sdk/core/typespec_macros/tests/data/safe-debug-tests/Cargo.toml	Converts a bench entry into a feature-gated test entry.
sdk/core/typespec_client_core/src/stream/tokio.rs	Adds Tokio FileReader implementing SeekableStream + AsyncRead/AsyncSeek.
sdk/core/typespec_client_core/src/stream/mod.rs	Adds new stream modules and makes SeekableStream::len/is_empty async.
sdk/core/typespec_client_core/src/stream/file_stream.rs	Adds buffered FileStream<T> implementing AsyncRead/AsyncSeek/SeekableStream.
sdk/core/typespec_client_core/src/stream/bytes_stream.rs	Adds AsyncSeek for BytesStream and Body conversion.
sdk/core/typespec_client_core/src/http/request/mod.rs	Makes Body::len/is_empty async; custom Clone for Body; adjusts tests.
sdk/core/typespec_client_core/src/http/policies/transport.rs	Updates empty-body content-length logic to await async is_empty().
sdk/core/typespec_client_core/src/http/policies/retry/mod.rs	Updates retry tests for async + adds coverage for retry body reset behavior.
sdk/core/typespec_client_core/src/http/clients/reqwest.rs	Minor refactor around body cloning for reqwest request building.
sdk/core/typespec_client_core/src/http/clients/mod.rs	Removes outdated doc text about implementers cloning request internals.
sdk/core/typespec_client_core/examples/core_binary_data_request.rs	Updates example to use new FileReader/FileStream approach.
sdk/core/typespec_client_core/examples/common/mod.rs	Removes old example support module.
sdk/core/typespec_client_core/examples/common/fs.rs	Removes old custom file streaming implementation used by examples.
sdk/core/typespec_client_core/Cargo.toml	Extends tokio feature to include tokio/fs; wires examples/feature flags.
sdk/core/typespec_client_core/CHANGELOG.md	Documents new stream adapters and the async len/is_empty breaking change.
sdk/core/azure_core_test/src/stream.rs	Updates test stream implementation to async SeekableStream::len.
sdk/core/azure_core/src/http/policies/auth/bearer_token_policy.rs	Updates auth challenge retry test to async body length checks.
sdk/core/azure_core/benches/benchmarks.rs	Adjusts benchmark request construction for updated request/body behavior.
sdk/core/azure_core/CHANGELOG.md	Mirrors TypeSpec client core changelog entries for stream/body changes.
Cargo.lock	Adds clap to dependency graph.
.vscode/cspell.json	Dictionary tweaks (ordering + added words).

Comments suppressed due to low confidence (1)

sdk/core/typespec_client_core/src/http/request/mod.rs:289

• RequestContent stops deriving Clone when json is disabled, while the json-enabled version is still Clone. Since the struct only contains Body + PhantomData, this looks like an accidental API regression and can break downstream code that relies on RequestContent: Clone in --no-default-features/--no-json builds. Consider restoring #[derive(Clone, Debug)] (or adding a manual impl Clone) for the non-json variant for API consistency.

/// The body content of a service client request.
///
/// This allows callers to pass a model to serialize or raw content to client methods.
#[cfg(not(feature = "json"))]
#[derive(Debug)]
pub struct RequestContent<T, F> {
    body: Body,
    phantom: PhantomData<(T, F)>,
}

[heaths]

Rebased to keep this somewhat current as I work on a separate PR.

[heaths]

Replaced by #4057