docs: fix non-compiling Rust session-limits example

[examon]

Summary

The Rust example in the "Configure a session limit" section of
docs/features/session-limits.md did not compile: it called
SessionConfig::new(), which does not exist. SessionConfig is constructed with
SessionConfig::default() plus builder methods (the crate uses default()
throughout; there is no SessionConfig::new). This one-line change makes the
documented snippet compile.

Fixes #2081

Change

docs/features/session-limits.md, Rust tab:

-        SessionConfig::new()
+        SessionConfig::default()

Root cause

SessionConfig (rust/src/types.rs) implements Default plus builder methods
but has no new associated function, so SessionConfig::new() fails with
error[E0599]. The snippet was a mistranslation of the sibling
TypeScript/Go/Java/.NET tabs, which all construct-then-configure. The block
carries <!-- docs-validate: skip --> and Rust is not in the doc-validation
language set, so CI never compiled it. The neighboring resume example already
uses the real ResumeSessionConfig::new(session_id) (a different type) and is
left unchanged.

Verification

Reproducer against the published crate:

Cargo.toml:

[dependencies]
github-copilot-sdk = "*"
tokio = { version = "1", features = ["full"] }

src/main.rs: the documented snippet wrapped in the minimal
#[tokio::main] async fn main the guide implies.

• Before: cargo build fails with
  error[E0599]: no associated function ... named 'new' found for struct 'SessionConfig'.
• After (SessionConfig::new() -> SessionConfig::default()): cargo build
  finishes clean.

[Copilot]

Pull request overview

Fixes #2081 by correcting the non-compiling Rust session-limits example.

Changes:

• Replaces nonexistent SessionConfig::new() with supported SessionConfig::default().

Show a summary per file

File	Description
docs/features/session-limits.md	Corrects Rust session construction.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Medium