[Jimy][mcp] Allow metadata to be passed from tool call - #19

`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.