Update testing docs to reference Bun test and preload setup

[zz-plant]

Motivation

• Documentation still referred to Vitest and an absent vitest.config.ts, which could confuse contributors running tests.
• Align docs with the repository's actual test tooling and preload setup to make local testing instructions accurate.

Description

• Updated docs/testing-todos.md to replace Vitest references with bun test and to describe the bunfig.toml preload of src/test/preload.ts for happy-dom and Astro virtual module mocks.
• Reworded docs/diagnostics-capacity-forecaster.md to reference Bun test coverage for the projection engine tests.
• Ran Prettier formatting on the edited docs with bunx prettier --write to keep style consistent.

Testing

• This is a docs-only change so no unit or E2E test suites were run and bun run check was intentionally skipped.
• Prettier formatting was executed with bunx prettier --write and completed without modifying content beyond the intended edits.

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[gemini-code-assist]

Summary of Changes

Hello @zz-plant, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request updates the project's documentation to accurately reflect the current testing infrastructure, transitioning all references from Vitest to Bun test. The changes provide clear and correct instructions for contributors on how to run tests and understand the project's test setup, including details on Bun's configuration and preload mechanisms. This ensures that the documentation remains aligned with the actual development environment, preventing confusion and streamlining the testing process for developers.

Highlights

• Testing Documentation Update: Replaced all references to Vitest with Bun test across the testing documentation to reflect the current tooling.
• Bun Test Configuration Clarification: Detailed the Bun test setup, including its configuration via bunfig.toml and the preloading of src/test/preload.ts for happy-dom and Astro virtual module mocks.
• Diagnostic Document Alignment: Updated docs/diagnostics-capacity-forecaster.md to correctly state Bun test coverage for the projection engine tests.
• Code Formatting: Applied Prettier formatting to the modified documentation files to maintain consistent style.

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[gemini-code-assist]

Code Review

This pull request updates the testing documentation to replace references to Vitest with bun test, aligning the docs with the project's current tooling. The changes in docs/testing-todos.md and docs/diagnostics-capacity-forecaster.md are accurate and improve clarity. I have provided one suggestion for docs/testing-todos.md to re-introduce the command for running tests in watch mode, as this is a common developer workflow that was mentioned in the previous version of the documentation.

[gemini-code-assist]

The previous version of the documentation mentioned running tests in watch mode, which is a very common development workflow. This information was lost in the update. It would be helpful to include how to run tests in watch mode with bun test for completeness.

Suggested change

	modules. Run `bun test` for local coverage and `bun run test:unit:ci` for a full run with the lcov
	reporter.
	modules. Run `bun test` for local coverage or `bun test --watch` for watch mode. Use `bun run test:unit:ci`
	for a full run with the lcov reporter.