Rust: Add example queries

[geoffw0]

Add example CodeQL queries for Rust, and link them from rust-further-reading.rst - similarly to how we have in other languages. These should help developers who are trying to write their first Rust queries.

[Copilot]

Pull Request Overview

This PR adds three example CodeQL queries for Rust to help developers learn query writing. The examples demonstrate different query patterns, from simple AST matching to more complex taint tracking scenarios.

• Adds three example queries: empty-if, simple-sql-injection, and simple-constant-password
• Creates a new qlpack structure for Rust examples
• Updates documentation to include links to the new example queries

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
rust/ql/src/change-notes/2025-11-07-example-queries.md	Documents the addition of the three new example queries
rust/ql/examples/snippets/empty_if.ql	Example query demonstrating basic AST pattern matching for empty if expressions
rust/ql/examples/snippets/simple_sql_injection.ql	Example query demonstrating taint tracking from user input to SQL query construction
rust/ql/examples/snippets/simple_constant_password.ql	Example query demonstrating taint tracking from string literals to password parameters
rust/ql/examples/qlpack.yml	Configuration file for the new rust-examples qlpack
rust/ql/examples/qlpack.lock.yml	Lock file for the rust-examples qlpack dependencies
docs/codeql/reusables/rust-further-reading.rst	Adds reference link to the new example queries

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