bug(api): MCP tool invoke drops OAuth Authorization when any custom header is set

从 api/core/tools/mcp_tool/tool.py 中的 MCPTool.invoke_remote_mcp_tool 开始，然后将其 header 处理与 api/services/tools/mcp_tools_manage_service.py 中的 _prepare_auth_headers 进行比较。使用自定义的 X-Tenant header 和已存储的 OAuth token 重现该单元测试用例，并验证调用会保留自定义 header、包含 Authorization，同时不会导致列表功能回归。

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Dify version

main (current)

Cloud or Self Hosted

Both

Problem

MCP tool listing and MCP tool invoke prepare Authorization differently.

• MCPToolManageService.list_provider_tools uses _prepare_auth_headers(), which always merges OAuth tokens into the request headers (custom headers + OAuth coexist).
• MCPTool.invoke_remote_mcp_tool gated token injection behind if not headers:. Any non-empty custom header map (for example X-Tenant: abc) skipped retrieve_tokens() entirely.

Falsifiable consequence:

1. Configure an MCP provider with at least one custom header and complete OAuth (tokens stored).
2. Refresh / list tools → succeeds (Authorization present via _prepare_auth_headers).
3. Invoke the same tool from Agent / Workflow → request goes out without Authorization, typically 401 from the MCP server, while listing still works.

This is an auth/header state asymmetry between list and invoke, not a UI issue.

Relevant code (before fix):

• api/services/tools/mcp_tools_manage_service.py → _prepare_auth_headers
• api/core/tools/mcp_tool/tool.py → invoke_remote_mcp_tool (if not headers: gate)

Steps to reproduce

Minimal unit repro (fails on main):

# decrypt_headers returns {"X-Tenant": "abc"}
# retrieve_tokens returns OAuthTokens(access_token="oauth-access-token", token_type="bearer", ...)
# invoke_remote_mcp_tool(...)
# MCPClientWithAuthRetry receives headers WITHOUT Authorization

Or manually:

1. Add MCP server with custom header X-Tenant=abc and OAuth credentials.
2. Click to fetch tools (list succeeds).
3. Call the tool from a workflow/agent → auth failure / 401.

Expected Behavior

Invoke must apply the same header merge as listing: custom headers kept, OAuth Authorization still attached when tokens exist.

Actual Behavior

Invoke drops OAuth whenever any custom header is present.

I am preparing a focused fix with regression tests and will open a PR linking this issue.