feat(http): send quicknode-cli user agent on all requests (DX-5655)

[johnpmitsch]

Summary

Every qn request previously went out with the SDK's auto-generated User-Agent (quicknode-sdk-rust/<version> (...)), so CLI traffic was indistinguishable from direct SDK usage. All requests now carry:

quicknode-cli/<version> (<os>-<arch>)     e.g. quicknode-cli/0.1.3 (macos-aarch64)

mirroring the SDK's own UA shape, and set via the SDK's supported override: custom HttpConfig.headers replace SDK-managed headers of the same name (see the SDK README section "Custom headers and User-Agent").

• New context::user_agent() builds the UA from CARGO_PKG_VERSION + std::env::consts::{OS, ARCH}, the same inputs the SDK uses.
• New shared context::sdk_config(api_key) constructor applies the header at every SDK construction site — the main Ctx path and both auth paths (login key validation, whoami) — so no traffic escapes attribution.
• SDK HTTP defaults (timeout, connection pooling) are untouched.

Test plan

• requests_carry_the_cli_user_agent (tests/endpoint.rs): wiremock asserts the exact User-Agent header reaches the wire on qn endpoint list, .expect(1).
• user_agent_identifies_the_cli: UA starts with quicknode-cli/ and contains the crate version.
• sdk_config_sets_the_user_agent_header_and_nothing_else: header wired, timeout_secs/pool_max_idle_per_host stay None.
• Full suite: cargo test green, cargo clippy --all-targets -- -D warnings clean, cargo fmt --check clean, release build clean.

Closes DX-5655