fix(kimi): tag content parts so serde emits the type discriminant

[flaviomartil]

Addresses #98.

Problem

KimiUserContentPart and KimiToolResultPart are declared #[serde(untagged)], so they serialize without a discriminant:

{"text": "..."}
{"image_url": {"url": "..."}}

Kimi expects the OpenAI multimodal shape and rejects the payload:

the message at position 0 with role 'user' contains an invalid part type:
the message at position 2 with role 'tool' contains an invalid part type:

The value after the colon is empty because the discriminant is missing.

The practical impact is that subagents do not work through the proxy. Claude Code's Task tool returns a multi-block tool_result, which always takes this path. Single-output tools such as Bash and Read are unaffected, because tool_result_content collapses a lone text part back into a plain string before serializing.

Change

Tag both enums:

#[serde(tag = "type", rename_all = "snake_case")]

Output becomes {"type":"text","text":"..."} and {"type":"image_url","image_url":{...}}.

Tests

Two regression tests added, one per enum, asserting the discriminant is present:

• user_content_parts_carry_a_type_discriminant
• tool_result_parts_carry_a_type_discriminant

The existing translate_tool_result_with_image, translate_tool_result_with_unsupported_blocks and translate_user_text_and_image_collapse_correctly assert only on the text and image_url keys, so they are unaffected.

Verification

Built and tested against a live Kimi account on macOS arm64. Before the change, a multimodal user message and any subagent invocation both returned invalid part type. After the change, both succeed.

[Copilot]

Pull request overview

This PR fixes Kimi request translation so multimodal user content and multi-part tool results serialize with an explicit "type" discriminant, matching the OpenAI-style content-part shape that Kimi expects (and preventing invalid part type: errors that break subagents and multimodal messages).

Changes:

• Switch KimiUserContentPart and KimiToolResultPart from #[serde(untagged)] to #[serde(tag = "type", rename_all = "snake_case")] so serialized parts include "type": "text" / "type": "image_url".
• Add regression tests verifying "type" is present for both user content parts and tool result parts.

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[raine]

Thanks!