fix(acp): preserve file source location when converting text resource blocks

[adityachaudhary99]

Issue for this PR

Closes #32558

Type of change

• Bug fix

What does this PR do?

When an ACP client (e.g. Zed) attaches a selected snippet as a resource content block with inline text, opencode was silently dropping the resource's uri. The snippet arrived as a bare text part with no provenance — the LLM and session replays had no way to trace it back to its source file or line range.

This PR adds a fileSourcePrefix helper that, when a text resource block carries a file:// uri, prefixes the text with the resolved file path (and line range when the uri contains a #L12 or #L12-15 fragment):

[/abs/path/to/app.ts:12-15]
const x = 1

Non-file:// uris (e.g. https://) and unparseable uris fall through unchanged — the helper returns undefined and the original text passes through untouched.

The Chat Completions (/resource_link) path is left unchanged.

How did you verify your code works?

Added three focused unit tests to packages/opencode/test/acp/content.test.ts:

• "resource with text prefixes the file source location" — verifies the file path prefix is prepended
• "resource with text includes the line range from the uri fragment" — verifies #L12-15 fragment parses to :12-15 in the prefix
• "resource with text and a non-file uri is left untouched" — verifies https:// uris pass through unchanged

Test URIs are constructed via path.resolve + pathToFileURL so they round-trip correctly on all platforms (Windows and POSIX). All 15 tests in the content suite pass.

Screenshots / recordings

N/A — no UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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