[Deps] Put clap behind an optional cli feature (#25)

`clap` sits in `[dependencies]` but only `bin/server.rs` and
`bin/client.rs` use it - nothing under `src/` mentions it. Every library
consumer of `mcp_rs` has therefore been compiling clap and its subtree
for nothing. This makes it optional behind a non-default `cli` feature
and gates the two binaries on it.

`required-features` means cargo skips a binary rather than failing when
the feature is off, so `cargo build` on a checkout still succeeds, and
both CI jobs already pass `--all-features`, so the binaries stay built
and linted.

Measured on this branch: `cargo tree -e normal` goes from 13
clap-subtree crates to zero, and `cargo tree -e normal --features cli`
still shows them.

In gitar this subtree reaches most of the workspace, since `mcp_rs` is a
dependency of `agent-models`. We just moved every gitar CLI from clap to
`usage`, and this was the one remaining edge keeping clap in the graph.

Heads up on two things I did not touch:

**CI is already red on main.** `cargo test --all-features` fails with 15
`E0061`s in `tests/tools.rs`, from `agent_id: Uuid` landing on
`CallToolArgs` as a required `TypedBuilder` field without the test call
sites being updated. I reproduced that on `main` at `9b4756f` with this
branch stashed, so it predates this PR.

Left alone, as the fix is a semantic call: either the tests supply an
`agent_id` or the field takes a `#[builder(default)]`, and whoever wrote
it knows which.

**gitar is pinned to a commit that is not on any branch.** Our `rev =
"43eb26e6"` is the pre-squash head of #19, reachable only through
`refs/pull/19/head`. #19 squash-merged as `b496f16`, and main has three
commits since.

So consuming this means moving off that dangling head onto main, which
also brings `e6409b7` (sync `get_tool`) and `9b4756f`. Separate bump on
our side, not something this PR does.

Author: kageiit

Cargo.toml

@@ -28,7 +28,9 @@ async-stream = "0.3"
 mime_guess = "2"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 config = "0.15"
-clap = { version = "4", features = ["derive"] }
+# Optional: the two binaries below are the only clap users, and a library consumer that
+# never builds them was compiling clap plus its 12-crate subtree for nothing.
+clap = { version = "4", features = ["derive"], optional = true }
 reqwest = { version = "0.12", features = ["json"] }
 reqwest-eventsource = "0.6"
 mime = "0.3"
@@ -38,13 +40,21 @@ tracing-core = "0.1"
 async-recursion = "1"
 http = "1.1"
 
+# Off by default so `mcp_rs` as a dependency carries no CLI parser. `required-features`
+# makes cargo skip the binaries rather than fail when it is off, so `cargo build` on a
+# checkout still succeeds and `--all-features` (what CI runs) still builds and lints them.
+[features]
+cli = ["dep:clap"]
+
 [[bin]]
 name = "server"
 path = "bin/server.rs"
+required-features = ["cli"]
 
 [[bin]]
 name = "client"
 path = "bin/client.rs"
+required-features = ["cli"]
 
 [lib]
 name = "mcp_rs"