fix(streamble_http) SendNotification not work bug

[Robin-ZMH]

Description

Fixes #334
The Reason that MCP Inspector and Python-SDK client can't receive sse stream is that the client will ignore the meesage when Status == 202.
According to https://modelcontextprotocol.io/specification/2025-06-18/basic/transports#sending-messages-to-the-server, the 5th and 6th is the spec of handle the input of JSON-RPC request, there no mention that we need to add StatusCode=202 to the response header.
After remove it, MCP Inspector can receive progress notifications as expected.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• MCP spec compatibility implementation
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring (no functional changes)
• Performance improvement
• Tests only (no functional changes)
• Other (please describe):

Checklist

• My code follows the code style of this project
• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works
• I have updated the documentation accordingly

Summary by CodeRabbit

• Bug Fixes
  • Updated server responses for server-sent events (SSE) to return HTTP 200 (OK) instead of 202 (Accepted).
  • Adjusted related test cases to expect the new HTTP status code for SSE responses.

[coderabbitai]

"""

Walkthrough

This change updates the HTTP status code in the handlePost method of the streamable HTTP server for SSE responses from 202 (Accepted) to 200 (OK). The corresponding test is modified to expect the 200 status code for SSE responses.

Changes

File(s)	Change Summary
server/streamable_http.go	Changed HTTP status code from 202 to 200 in SSE response handling in handlePost method.
server/streamable_http_test.go	Updated test assertion to expect HTTP 200 instead of 202 for SSE response in "response with sse".

Assessment against linked issues

Objective	Addressed	Explanation
Ensure SendNotificationToClient results in notifications being received by client (#334)	✅

Assessment against linked issues: Out-of-scope changes

No out-of-scope changes found.

Possibly related PRs

• fix(srv/stream): correct handleGet status code to 200 #379: Changes HTTP status code from 202 to 200 in the handleGet method for SSE responses, similar to the current PR's change for handlePost.
• fix: panic when streamable HTTP server sends notification #348: Adds SSE upgrade logic and notification handling in the streamable HTTP server, related to SSE handling but different focus.

Suggested reviewers

• pottekkat
  """

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📒 Files selected for processing (1)

• server/streamable_http.go (2 hunks)

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• server/streamable_http.go

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[dugenkui03]

Thanks for your contribution. could you please replace http.StatusAccepted with http.StatusOK?