feat(mcp): add context length guard to prevent oversized requests

[shinchan-zhai]

Problem

When nofx calls AI models via claw402, messages can exceed model context limits (e.g. DeepSeek 131K), causing 400 errors:

This model's maximum context length is 131072 tokens. However, you requested 154604 tokens

Solution

• New MaxContext config field (default 0 = no limit, backward compatible)
• Token estimation (~3 chars/token heuristic for mixed CJK/English)
• Auto-truncation of oldest non-system messages when limit exceeded
• Supports both map[string]string and map[string]any message formats
• Preserves all system messages + at least 1 non-system message
• Logs warning when truncation occurs

Usage

client := mcp.NewDeepSeekClient(mcp.WithMaxContext(131072)) // 128K

Files Changed

• mcp/config.go — add MaxContext field
• mcp/options.go — add WithMaxContext() option
• mcp/context_guard.go — token estimation + truncation (both message types)
• mcp/client.go — integrate guard in BuildMCPRequestBody + BuildRequestBodyFromRequest
• mcp/context_guard_test.go — 6 test cases, all passing

[cla-assistant]

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[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[github-actions]

🤖 Advisory Check Results

These are advisory checks to help improve code quality. They won't block your PR from being merged.

📋 PR Information

Title Format: ✅ Good - Follows Conventional Commits
PR Size: 🟡 Medium (311 lines: +311 -0)

🔧 Backend Checks

Go Formatting: ⚠️ Needs formatting

Files needing formatting

api/strategy.go
api/utils_test.go
config/config.go
kernel/formatter.go
kernel/grid_engine.go
kernel/validate_test.go
main.go
manager/trader_manager.go
market/data.go
market/types.go

Go Vet: ✅ Good
Tests: ✅ Passed

Fix locally:

go fmt ./...      # Format code
go vet ./...      # Check for issues
go test ./...     # Run tests

⚛️ Frontend Checks

Build & Type Check: ✅ Success

Fix locally:

cd web
npm run build  # Test build (includes type checking)

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Questions? Feel free to ask in the comments! 🙏

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