Fix PR #812 review issues

[Mshehu5]

This PR addresses the review comments from @nothingmuch and @0xBEEFCAF3 on PR #812 (Deduplicate payjoin URI creation logic)

Changes Made

• Import Cleanup: Add direct import of bitcoin_uri::Uri instead of using relative imports
• Output Substitution Fix: Fix output substitution handling to properly default to enabled for v2 and disabled for v1

Implementation Details

The pj_uri function in SessionHistory now follows a clearer flow:

1. Find the session context: First locates the session context from the Created event in the history
2. Determine version and set output substitution: Checks if there's an UncheckedProposal event to determine the protocol version, then sets output substitution to disabled for v1 and enabled for v2
3. Generate the URI: Creates the Payjoin URI with the appropriate context and output substitution setting