feat: browser_file_download

[Skn0tt]

Resolves #154

[Copilot]

Pull Request Overview

This PR adds support for a new browser file download tool, alongside accompanying tests and updated type definitions. Key changes include:

• Introducing a TestServer class to facilitate serving files during tests.
• Adding new tests for browser file download functionality, including both HTML and PDF links.
• Extending the tool definitions and context handling to support a download modal state.

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/fixtures.ts	Introduces a TestServer class for managing a local HTTP server.
tests/files.spec.ts	Adds tests verifying browser file download behavior.
src/tools/tool.ts	Updates modal state types to include download state.
src/tools/files.ts	Implements the downloadFile tool and its schema for file downloads.
src/context.ts	Adds modal state tracking and handling for download events.

Comments suppressed due to low confidence (1)

tests/fixtures.ts:28

• [nitpick] Consider renaming the instance property 'PREFIX' to 'prefix' to indicate that it is not a constant.

PREFIX: string;