fix(clients): align client version constants to package manifests (CLI-18/21/26/29)

- .NET: add explicit <Version>0.1.2</Version> to client csproj (CLI-18)
- Go: ClientVersion 0.1.0-dev -> 0.1.2 (CLI-21)
- Python: __version__ 0.1.0 -> 0.1.2 to match pyproject.toml (CLI-26)
- Rust: CLIENT_VERSION now sourced from env!(CARGO_PKG_VERSION) so it
  cannot drift from Cargo.toml (CLI-29)

Verified: dotnet build clean; go build/test ok; cargo check/test/clippy
clean; pytest 127 passed. P2 client version-drift cluster.

Claude-Session: https://claude.ai/code/session_01DMXXvNuPekkkrTEyPNxEkW

clients/rust/src/version.rs

@@ -3,8 +3,9 @@
 //! The protocol versions track the values the gateway and worker negotiate on
 //! `OpenSession` and let test harnesses cross-check the wire contract.
 
-/// Semantic version of this Rust client crate. Mirrors `Cargo.toml`.
-pub const CLIENT_VERSION: &str = "0.1.0-dev";
+/// Semantic version of this Rust client crate. Sourced from `Cargo.toml` at
+/// compile time so the two cannot drift.
+pub const CLIENT_VERSION: &str = env!("CARGO_PKG_VERSION");
 
 /// Public gateway gRPC protocol version this client targets.
 pub const GATEWAY_PROTOCOL_VERSION: u32 = 3;