docs: add Rust and TypeScript module unit testing guide

[clockwork-labs-bot]

Description of Changes

Add a unit-testing how-to for Rust and TypeScript modules, with links from both quickstarts and Developer Resources.

• Explain the 2.12.0 minimum version, native Rust setup (test-utils), and Node/Vitest setup with spacetimedbModuleTestPlugin.
• Provide runnable, language-appropriate message-module examples testing caller identity, deterministic timestamps, and transaction rollback.
• Cover internal versus JWT-based test auth, fixture isolation, explicit lifecycle setup, deterministic RNG, and procedure/view/query contexts.
• Show outbound HTTP mocks in both languages, including TypeScript response bodies and the Rust unstable feature requirement.
• Distinguish module unit tests from integration tests against a running server.

Source references

• Module unit testing: Module unit testing #5558
• Published-package Vitest setup and HTTP response body fixes: Fix two TypeScript module test harness problems #6139
• Rust example: https://github.com/clockworklabs/SpacetimeDB/blob/master/templates/chat-console-rs/spacetimedb/src/lib.rs
• TypeScript example: https://github.com/clockworklabs/SpacetimeDB/blob/master/templates/chat-react-ts/spacetimedb/src/index.test.ts

API and ABI breaking changes

None. Documentation only.

Rollback safety impact

n/a

Expected complexity level and risk

1. New guide and three navigation/cross-link updates; no product or test-runtime changes.

Testing

Examples are extracted directly from the guide and run in temporary, standalone projects using the published spacetimedb 2.12.0 packages, not repository-only imports.

• TypeScript: all 3 extracted Vitest tests passed, including reading the mocked HTTP body.
• TypeScript: strict tsc --noEmit passed for module, tests, and Vitest config.
• pnpm --dir docs typecheck passed.
• Full pnpm --dir docs build passed, including broken-link checks. It emitted the existing Browserslist age and ask-ai Markdown conversion warnings.
• Rust: all 3 extracted tests passed with test-utils,unstable; the 2 reducer tests also passed with only test-utils.
• Final pnpm --dir docs build passed again after the dependency-workaround note was added.
• git diff --cached --check passed; TypeScript examples use camelCase.

Native Rust dependency finding

A clean lockfile against crates.io currently selects ICU 2.3, which fails while compiling the transitive temporal_rs 0.1.2 dependency (including a missing AnyCalendarDifferenceError). The repository lockfile uses ICU 2.1.1. Verified that pinning icu_calendar and icu_locale to =2.1.1 in dev-dependencies resolves the failure; the guide includes that narrowly scoped troubleshooting workaround. This is an upstream/runtime dependency issue, not a failure in the example module. No product dependency changes are included in this docs PR.