#17885: fix: replace anyOf union type in image tool schema

Fixes #17868 ## Problem The `image` tool parameter used `Type.Union([Type.String(), Type.Array(Type.String())])` which generates an `anyOf` in JSON Schema. Claude API rejects `anyOf` in tool schemas. ## Solution Replace with `Type.String()`. The runtime normalization already handles both string and array inputs gracefully. ## Changes - `src/agents/tools/image-tool.ts`: Replace `Type.Union([Type.String(), Type.Array(Type.String())])` with `Type.String()` <!-- greptile_comment --> <h3>Greptile Summary</h3> Replaced `Type.Union` with `Type.String()` in the `image` tool parameter schema to fix Claude API compatibility (which rejects `anyOf` schemas). The runtime normalization (src/agents/tools/image-tool.ts:387-397) already handles both string and array inputs gracefully, so this change only affects the JSON Schema definition without impacting functionality. - Aligns with repository style guide (AGENTS.md:179) that prohibits `Type.Union` in tool schemas - Runtime continues to accept both string and array inputs via existing normalization logic - Fixes issue #17868 <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk - The change is minimal (one line), well-documented, and follows repository guidelines. The runtime normalization code ensures backward compatibility with both string and array inputs. The fix addresses a concrete API compatibility issue and aligns with established coding patterns in the repository. - No files require special attention _{Last reviewed commit: 79feb94} <!-- greptile_other_comments_section --> _{(5/5) You can turn off certain types of comments like style [here](https://app.greptile.com/review/github)!} **Context used:** - Context from `dashboard` - CLAUDE.md ([source](https://app.greptile.com/review/custom-context?memory=fd949e91-5c3a-4ab5-90a1-cbe184fd6ce8)) <!-- /greptile_comment -->