ci: add cargo fmt --check and cargo deny to CI

[cyber-excel10]

Closes #129

Changes

• Added cargo fmt --all --check as a separate CI job to catch formatting issues
• Added cargo deny check via EmbarkStudios/cargo-deny-action@v2 for advisory and license scanning
• Added deny.toml at repo root with sensible defaults (version = 2 schema):
  • Advisories: known transitive issues via pinned ureq =2.7.1 are documented and ignored with comments
  • Licenses: explicit allowlist of permissive FOSS licenses + ring clarification for missing license field
  • Bans: duplicate versions warn, wildcards denied
  • Sources: crates.io only

Pre-existing issues fixed

• Removed duplicate pub mod template; in src/utils/mod.rs (file doesn't exist, only templates.rs does)
• Removed duplicate struct body in src/utils/horizon.rs that caused a parse error blocking cargo fmt
• Auto-formatted benches/benchmarks.rs and tests/template_marketplace_test.rs via cargo fmt --all

All three CI jobs run in parallel for faster feedback.

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[cyber-excel10]

Hi, @Nanle-code based on the failing Build, Test & Clippy job:

These build errors are pre-existing on main before this PR was opened. My PR only addresses issue #129 — adding cargo fmt --check and cargo deny to CI. The Rustfmt and Cargo Deny jobs that i added by this PR both pass successfully.