fix(agent-core-v2): fall back to protocol default base URL when unconfigured

[sailist]

Related Issue

No linked issue — the problem is explained below.

Problem

In agent-core-v2, a structured model whose provider has no base_url configured (neither in config, provider, nor env) was rejected with CONFIG_INVALID ("missing a base URL"). This is a regression from v1, where provider-manager let providers without a configured base_url fall back to their protocol's official default endpoint (e.g. the official Anthropic / OpenAI endpoints). Users migrating such configs to v2 hit a hard config error instead of the previous working behavior.

What changed

• ModelResolverService: removed the CONFIG_INVALID throw when a structured provider resolves no base URL; resolvedBaseUrl is now string | undefined and the wire provider applies its protocol default endpoint, matching v1's provider-manager.
• Guarded the Anthropic /v1 stripping so it only runs when a base URL actually exists.
• Threaded the optional baseUrl through the Model, ModelImpl, ProtocolAdapterConfig, and buildProtocolProviderOptions types.
• Added tests: every protocol type (anthropic, openai, openai_responses, kimi, google-genai) without base_url resolves to undefined; an anthropic-protocol override without base_url is left untouched; and flat models with neither providerId nor baseUrl are still rejected as before.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

[changeset-bot]

🦋 Changeset detected

Latest commit: ba87e6c

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 4 packages

Name	Type
@moonshot-ai/agent-core-v2	Patch
@moonshot-ai/kimi-code	Patch
@moonshot-ai/kap-server	Patch
@moonshot-ai/klient	Patch

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commit: ba87e6c