feat(session): handle SSE responses in fetch

[parvahuja]

Summary

• let selected client methods adapt successful paid responses through an internal hook
• retry Tempo session streams after channel-open acknowledgements and handle voucher and top-up frames in-band
• preserve application SSE frames, response metadata, cancellation, public method types, and existing SessionManager behavior

Why

Fetch.from completed the initial session challenge but could return the channel-open acknowledgement or expose later payment frames to callers. This completes the session lifecycle at the fetch boundary and returns a normal application Response.

[pkg-pr-new]

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npm i https://pkg.pr.new/mppx@658

commit: a5b80a3