Fix rustls for out-of-the-box use

[alawrenc]

1. Reset the default to use the system TLS for full backwards compatability.
2. Enable a crate feature for Rustls that automatically registers a provider as well.
3. Uses secp256k1::rand::rng(); to avoid dependency collisions.

[Copilot]

Pull request overview

This PR adjusts the Rust SDK’s TLS setup to work out-of-the-box by defaulting to the platform-native TLS backend while optionally supporting rustls + aws-lc-rs, and reduces RNG dependency/version friction in secp256k1-related tests.

Changes:

• Introduce mutually-exclusive TLS feature flags (tls-native-tls default, optional tls-rustls) and add a one-time TLS runtime initializer for rustls provider installation.
• Update REST entrypoints to call ensure_tls_runtime() before making API requests.
• Update secp256k1 test RNG usage to use secp256k1::rand::rng() and expand CI to cargo check both TLS configurations.

Reviewed changes

Copilot reviewed 12 out of 13 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
rust/src/tls_runtime.rs	Adds process-wide TLS runtime initialization + feature gating for TLS backends.
rust/src/lib.rs	Wires TLS runtime module + re-exports ensure_tls_runtime via prelude.
rust/src/env.rs	Calls ensure_tls_runtime() before exchange info REST calls.
rust/src/authenticate.rs	Calls ensure_tls_runtime() before auth REST calls; updates secp256k1 test RNG usage.
rust/src/create_order.rs	Updates secp256k1 test RNG usage.
rust/src/withdraw.rs	Updates secp256k1 test RNG usage.
rust/src/update_leverage.rs	Updates secp256k1 test RNG usage.
rust/src/adjust_isolated_margin.rs	Updates secp256k1 test RNG usage.
rust/src/account_authorization.rs	Updates secp256k1 test RNG usage (authorize/deauthorize tests).
rust/Cargo.toml	Adds TLS feature flags; makes reqwest default-features=false; adds optional rustls dep; updates dev rand version.
rust/README.md	Documents TLS backend selection and how to enable rustls.
.github/workflows/typecheck.yaml	Triggers on Rust changes; adds cargo check for default + rustls feature sets.
rust/Cargo.lock	Updates lockfile for new reqwest/rustls/native-tls dependency graph.

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