Add streaming-tour and middleware examples

[iainmcgin]

Closes #45

Adds two small focused example crates that close the most-asked-about gaps for new adopters: protocol-mechanics demonstration of all four RPC types, and server-side tower middleware composition.

examples/streaming-tour

A trivial `NumberService` exercising every ConnectRPC RPC type:

RPC	Type	Semantics
`Square`	Unary	n -> n*n
`Range`	Server streaming	emit `count` integers from `start`
`Sum`	Client streaming	total all values
`RunningSum`	Bidirectional streaming	emit running total after each request

The methods are deliberately tiny - the example exists to show handler signatures and client invocation patterns side-by-side. Ships server + client binaries plus an integration test that spins up the server in-process and verifies every RPC.

examples/middleware

Server-side tower middleware composition around the connect router:

• `AuthLayer` - hand-rolled `tower::Layer` that validates a `Bearer ` header against a static map. On success, stamps a `UserId` into the request extensions. On failure, short-circuits with a 401 in the Connect-protocol JSON error shape.
• `TraceLayer` - tower-http request/response logging.
• `TimeoutLayer` - per-request handler deadline.

Layers compose via `ServiceBuilder` mounted on `axum::Router::layer()`, so axum handles body conversion from `ConnectRpcBody` to `axum::body::Body`. The handler reads `UserId` from `Context::extensions()`, performs a per-secret permission check, and writes a `x-served-by` response trailer via `Context::set_trailer()`.

The client demonstrates `ClientConfig::default_header` for the auth header, `ClientConfig::default_timeout` for a default deadline, and `CallOptions::with_timeout` for a per-call override. Integration tests cover four paths: authorized success (verifying the trailer arrives), missing auth header, invalid token, and permission denied.

Verification

• `cargo build -p streaming-tour-example -p middleware-example` clean
• `cargo test -p streaming-tour-example -p middleware-example` - 8/8 tests pass
• `cargo clippy -p streaming-tour-example -p middleware-example --all-targets -- -D warnings` clean
• `cargo +nightly fmt --all -- --check` clean
• `cargo check --workspace --all-features --all-targets` clean (workspace-wide)

Branch order note

This branch is off main (pre-#44). When #44 lands, I'll rebase this branch onto main and add `rust-version.workspace = true` to the two new examples' `Cargo.toml` to match the rest of the workspace.

[iainmcgin]

[claude code] Updated per review feedback: switched proto packages from connectrpc.tour.v1 / connectrpc.middleware_demo.v1 to anthropic.connectrpc.tour.v1 / anthropic.connectrpc.middleware_demo.v1 (matching the existing convention from examples/multiservice/), and switched both new .proto files from syntax = "proto3" to edition = "2023" with features.field_presence = IMPLICIT to keep proto3-equivalent semantics. (Edition 2024 requires protoc 30+; CI's apt-installed protoc and the local 29.6 toolchain only support up to edition 2023.) Tests still 8/8 passing.

[iainmcgin]

[claude code] Removed the IMPLICIT field-presence override, so the new examples now use edition 2023's actual defaults (EXPLICIT presence). All scalar/string fields are now Option<T> in generated Rust code. Source code updated accordingly: writes wrap in Some(...) and reads use .unwrap_or(...) to retain proto3-equivalent zero-default semantics. Tests still 8/8 passing.

[iainmcgin]

[claude code] Refactored the auth middleware from a hand-rolled tower::Layer/tower::Service pair to axum::middleware::from_fn_with_state. The connectrpc-specific lesson ("middleware writes to request extensions, handler reads via Context::extensions") is unchanged, but the AuthLayer/Service trait wiring is replaced with a plain async fn. Net delete of ~85 lines, same end-to-end behavior. Tests still 4/4 passing.

[iainmcgin]

[claude code] CI was failing because Ubuntu's apt-installed protoc (v21.x) doesn't recognize edition = "2023" syntax used by the new examples. Fixed by:

1. Rebasing onto main (which merged Pin MSRV to Rust 1.88 and verify in CI #44 with the MSRV pin)
2. Adding rust-version.workspace = true to the new example Cargo.tomls (matching the rest of the workspace post-Pin MSRV to Rust 1.88 and verify in CI #44)
3. Replacing apt install protobuf-compiler with curl-from-GitHub-release for protoc 33.5 in all 7 CI jobs that need it (matching the buffa CI pattern)

Local verification: cargo test -p streaming-tour-example -p middleware-example (8/8 pass), cargo +1.88 check --workspace --all-features --all-targets (clean), workflow YAML validates.

[asacamano]

Nice example!

[iainmcgin]

[claude code] CI green now (12/12 checks). The Documentation job was failing on a separate issue beyond the protoc version: a broken intra-doc link in generated client code.

The codegen template at connectrpc-codegen/src/codegen.rs:1115 emits a doc comment with the shortcut intra-doc link [connectrpc::client::ClientConfig]. When the proto package contains connectrpc as a path component (e.g. anthropic.connectrpc.tour.v1), the generated module proto::anthropic::connectrpc shadows the external crate name during rustdoc path resolution, producing 'no item named client in module connectrpc'.

Existing examples don't hit this because they ship checked-in code with use connectrpc::client::ClientConfig; at file top, so the doc comment can use the short [ClientConfig] form. Build.rs-generated code (no top-level use statements) needs the absolute-path form.

Fixed in the codegen by switching to the reference-style [ClientConfig](::connectrpc::client::ClientConfig) with a leading ::, matching the pattern already used for UnaryResponse references on lines 685 and 694.