fix: accept unset Loom import options from existing clients

[richiemcilroy]

Summary

• Accept null for the optional owner and Space fields on the Loom import API, so installed CLI and MCP versions work after a web deployment.
• Omit unset fields from new CLI requests while preserving explicit owner and Space values.
• Cover omitted, null, supplied, and invalid option values in the API contract test.

Why

Existing CLI and MCP clients serialize absent options as JSON null. The API schema rejected that payload with HTTP 400 before the import handler ran. The handler already treats null values as unsupplied options.

Validation

• Focused web contract suite: 22 tests passed.
• Scoped Biome check and git diff --check passed.
• Earlier cargo fmt --all and cargo check -p cap passed for the CLI change. Its focused Rust test could not complete because the isolated worktree ran out of disk space during compilation.
• Local web typecheck in the isolated worktree was inconclusive because workspace package dist artifacts were absent; CI will run in its normal environment.

No production import was attempted. The server change requires web deployment; it does not require an installed CLI update.

Confidence Score: 5/5

The PR appears safe to merge with no actionable correctness, security, or repository-rule issues identified.

Summary

The PR fixes Loom imports from the CLI by conditionally adding optional owner and Space fields only when supplied, preventing absent options from being serialized as JSON null.

• Adds a dedicated payload-construction helper.
• Preserves explicitly provided owner and Space values.
• Adds regression coverage for payloads with both absent and supplied optional fields.

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