fix: allow custom Content-Type header in StreamableHTTPTransport

[guoyangzhen]

Fixes #2375

Problem

StreamableHTTPTransport._prepare_headers() hardcodes Content-Type: application/json, which overwrites any user-configured header (e.g. application/json; charset=utf-8). Users who pass custom headers via httpx.AsyncClient(headers=...) find them ignored because request-level headers take precedence over client defaults in httpx.

Solution

Add an optional headers parameter to both StreamableHTTPTransport.__init__ and streamable_http_client. User-supplied headers are merged after the MCP defaults, so they take precedence. This is backward-compatible — existing code with no headers argument behaves identically.

Usage

async with streamable_http_client(
    url,
    headers={'Content-Type': 'application/json; charset=utf-8'},
) as (read, write):
    ...

Changes

• StreamableHTTPTransport.__init__: new headers dict param, stored as self._user_headers
• _prepare_headers(): calls headers.update(self._user_headers) after setting defaults
• streamable_http_client(): new headers kwarg, forwarded to transport