fix: normalize multi-round gateway streaming

[harivilasp]

Closes #119.

Summary

• Add a gateway stream accumulator that owns client-visible SSE lifecycle de-duping, sequence_number assignment, and output_index rebasing across gateway rounds.
• Route upstream forwarded frames, synthetic web_search/MCP frames, and the terminal response event through the same stream accumulator.
• Add a multi-round streaming regression test covering a gateway tool round followed by a final text round.

Test Plan

• cargo fmt --check
• cargo test -p agentic-server-core --test web_search_tool_test multi_round_stream_has_single_lifecycle_and_monotonic_public_sequence
• cargo test -p agentic-server-core --test web_search_tool_test
• cargo test -p agentic-server-core
• cargo clippy --all-targets -- -D warnings
• cargo test

[harivilasp]

Cancelling this pickup; #121 has the broader test-coverage scope and this PR should not proceed separately.