fix: prevent SSE voucher posts from consuming charges

[brendanjryan]

Summary

• Prevented SSE session voucher management POSTs from being counted as billable content charges.
• Updated the SSE session example to use explicit per-token stream charges with the TIP-1034 session flow.
• Added a patch changeset for the session accounting fix.

Motivation

SSE voucher updates are management traffic. Counting them as content charges consumed voucher headroom before the stream could commit its reserved charge, which could terminate the stream.

Key design considerations

• Preserved non-SSE POST content accounting.
• Kept SSE content precharge behavior for GET streams.
• Removed default fee sponsorship from the example so the funded client pays channel-open gas.

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commit: a44475d