[lordcheng10]

Description

openapi-to-mcp currently appends serialized query parameters with an unconditional
?. Some OpenAPI documents, including S3-style APIs, use a query suffix in the path
template. For /{Bucket}?acl with Bucket=b and a declared prefix=logs query
parameter, the tool sends:

/b?acl?prefix=logs

The upstream therefore receives one query parameter named acl?prefix; prefix
is not present under its declared name.

The handler now checks whether the constructed URL already contains a query marker
and uses & in that case. Templates without a query suffix still use ?, so their
behavior is unchanged:

/{Bucket}       -> /b?prefix=logs
/{Bucket}?acl   -> /b?acl&prefix=logs

Which issue(s) this PR fixes:

Fixes #14161

Reproduction and evidence

Verified on APISIX 3.19.0 from master
7577c31733bf0f7067cda6af3153b2a56ec8c6f5. The running container's
apisix/plugins/openapi-to-mcp/tools/handler.lua had SHA-256
2f293159ca3469ec4f997eadd8ce9f668bc6a2b041296d68d50099209230054c, matching the
unmodified master file.

An openapi-to-mcp Route served an operation for /{Bucket}?acl. A JSON-RPC
tools/call with Bucket=b and prefix=logs reached the Node upstream as:

Before:

/b?acl?prefix=logs

After applying the patch:

/b?acl&prefix=logs

Tests

• Added a regression test covering both the existing no-suffix URL and the
  query-suffix merge.
• Verified the patched handler loads successfully with LuaJIT.
• git diff --check passes.
• The local machine does not have OpenResty, wget, luacheck, Test::Nginx,
  or t::APISIX; therefore make lint and the target .t could not run locally.
  The real-container reproduction above exercises the complete MCP tool-call path.

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible

No documentation update is required because this restores the documented OpenAPI
query-parameter behavior without adding configuration.

[lordcheng10]

PTAL @Baoyuantop @nic-6443 @AlinsRan @ruanweihong-aaa @shreemaan-abhishek. This is a minimal follow-up to the query-suffix behavior covered in #14154, with a real-container before/after reproduction for #14161.

[AlinsRan]

LGTM