chore: activate code coverage in bun test config

[didier-durand]

Hi,
In order to be able to contribute some revelant tests: i.e. where current code coverage is low, I updated bunfig.toml to activate code coverage.

This is a sample of what I get on my machine when running the tests included in repo:

----------------------------------------------------------------------------------------------------------|---------|---------|-------------------
File | % Funcs | % Lines | Uncovered Line #s
----------------------------------------------------------------------------------------------------------|---------|---------|-------------------
All files | 45.38 | 52.40 |
...
...
...
packages/opencode/src/provider/sdk/openai-compatible/src/responses/openai-responses-prepare-tools.ts | 0.00 | 3.57 | 13-174 packages/opencode/src/provider/sdk/openai-compatible/src/responses/tool/code-interpreter.ts | 0.00 | 91.89 | 84-86 packages/opencode/src/provider/sdk/openai-compatible/src/responses/tool/file-search.ts | 0.00 | 100.00 | packages/opencode/src/provider/sdk/openai-compatible/src/responses/tool/image-generation.ts | 0.00 | 91.43 | 111-113
packages/opencode/src/provider/sdk/openai-compatible/src/responses/tool/local-shell.ts | 100.00 | 100.00 | packages/opencode/src/provider/sdk/openai-compatible/src/responses/tool/web-search-preview.ts | 100.00 | 100.00 | packages/opencode/src/provider/sdk/openai-compatible/src/responses/tool/web-search.ts | 0.00 | 93.62 | 99-101
...
...
...
783 expect() calls
Ran 385 tests across 33 files. [33.23s]

Cheers,
Didier