fix(streamable_http): allow Content-Type header override

[haroldfabla2-hue]

Summary

Allow and to accept a custom parameter instead of hardcoding application/json. This enables clients to override the Content-Type header to include custom charsets (e.g. application/json; charset=utf-8) or other attributes required by specific server implementations.

Changes

• ****: Added parameter and stored it as
• ****: Changed hardcoded "content-type": "application/json" to use self.content_type
• ****: Added parameter, passed through to
• Added test file with 3 test cases

Fixes

Fixes #2375

Example usage

# Before (hardcoded, no way to override):
transport = StreamableHTTPTransport(url)

# After (custom charset supported):
transport = StreamableHTTPTransport(
    url,
    content_type="application/json; charset=utf-8",
)

# Or via the client helper:
async with streamable_http_client(
    url,
    content_type="application/json; charset=utf-8",
) as (read, write):
    ...