Rewrite mcp.longbridge.com to .cn for CN region build

[hold-baby]

Merge Verdict

[APPROVE] — Small, mechanical extension of the existing region rewrite to cover a third root domain (mcp.longbridge.com).

3 files · +8 / -1 · region.config.ts docs/.vitepress/

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Summary

• region.config.ts: RegionConfig interface gains a new required field mcpHostname; CN region sets it to https://mcp.longbridge.cn.
• docs/.vitepress/region-utils.ts: buildRegionUrlReplacements() emits a third pair of rules (URL + bare-text) for the MCP hostname — same shape as the existing siteHostname / apiBaseUrl blocks.
• docs/.vitepress/config.mts: MCP_TOOLS_URL (used by the fetch-mcp-tools build-time plugin) now resolves to ${regionCfg?.mcpHostname || 'https://mcp.longbridge.com'}/mcp/tools.json — CN builds source the tool catalogue from the regional endpoint instead of .com.

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Risk Analysis

Risk	Level	Mitigation
mcp.longbridge.cn/mcp/tools.json endpoint not yet live	🟡	Reviewer to confirm before merge — fetch-mcp-tools will hard-fail the CN build if the URL 404s
Global build (build:release) regression	✅	Verified: with no VITE_REGION, regionCfg is undefined → helper returns [] for MCP rules; MCP_TOOLS_URL falls back to mcp.longbridge.com; mcp.html keeps 7 .com + 1 .cn (original "China mainland" section) identical to source
Order-dependent rewriting between mcp / open / openapi	✅	All three hostnames are mutually non-substring (different 5th char or different prefix), so the three protocol-prefixed + three bare-text rules are independent regardless of order
Mainland users on global .cn site getting wrong fallback	✅	This PR only adds a CN-only rule; nothing about the global path changes

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Design Decisions

• Add mcpHostname to RegionConfig rather than hardcoding inside the helper — keeps the regional URL list in one declarative file (region.config.ts) instead of scattering string constants across region-utils.ts. Matches the pattern already set by siteHostname / apiBaseUrl.
• Switch the build-time fetch source, not just the displayed URL — chosen so the CN build's mcp-tools.json actually reflects the catalogue served by the CN MCP endpoint (potentially region-specific tools), instead of CN docs displaying .cn while embedded data was scraped from .com.
• Field is required, not optional — every region must declare its MCP hostname explicitly; prevents silent fallback to .com when a new region is added.

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Code Notes

1. [Needs review] Confirm https://mcp.longbridge.cn/mcp/tools.json is live and returns the same JSON shape as the global endpoint before merge. If not yet deployed, the CN build will fail at fetch-mcp-tools buildStart with an HTTP error.

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Verification

• ✅ bun run build:cn succeeds; rg -l '(open|openapi|mcp)\.longbridge\.com' docs/.vitepress/dist → zero residual .com across HTML / MD / JS / install scripts / mcp-tools.json.
• ✅ bun run build:release succeeds; dist/docs/mcp.md per-file domain distribution (1× mcp.longbridge.cn + 7× mcp.longbridge.com) matches source 1:1.
• ✅ Source install.ps1 is byte-identical to global build output (no accidental rewrite).