refactor: centralize model input token limit resolution

[NarwhalChen]

Related Issue

Follow-up to #2036. @RealKai42 identified the duplicated input-limit fallback as a deliberate out-of-scope cleanup.

Problem

Compaction, context validation, session status, and SDK/server projections independently repeated the rule that a model's prompt ceiling prefers max_input_tokens, falls back to max_context_tokens, and uses zero when capability data is unavailable. Keeping that semantic decision at each consumer risks the engine and its status surfaces drifting apart.

What changed

• Add getModelInputTokenLimit at the model-capability boundary in legacy kosong and in the self-contained v2 contract.
• Migrate the duplicated v1, v2, SDK, and kap-server consumers while preserving the existing v1/v2 dependency boundary.
• Cover input-cap precedence, context-window fallback, and unavailable capability data in both contract test suites.
• Add patch changesets for the bundled CLI and SDK artifacts.

Completion-budget code intentionally continues to use the total context window because it answers a different question: how much output can fit after the prompt.

Verification

• pnpm --filter @moonshot-ai/agent-core test — 3,978 passed
• pnpm --filter @moonshot-ai/agent-core-v2 test — 3,951 passed
• Relevant SDK and kap-server suites — 538 passed when run serially
• Typechecks passed for kosong, agent-core, agent-core-v2, kimi-code-sdk, and kap-server
• pnpm --filter @moonshot-ai/agent-core-v2 lint:domain
• oxlint --type-aware on all touched TypeScript files — 0 errors
• git diff --check

Checklist

• I have read the CONTRIBUTING document.
• I have linked the prior maintainer request and explained the problem above.
• I have added tests that prove the behavior works.
• Ran gen-changesets skill and added a changeset.
• No documentation update is needed because this centralizes existing internal behavior.

[changeset-bot]

🦋 Changeset detected

Latest commit: 1cf8f8c

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

This PR includes changesets to release 2 packages

Name	Type
@moonshot-ai/kimi-code	Patch
@moonshot-ai/kimi-code-sdk	Patch

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