Remove hard-coded output message cap

[MyTH-zyxeon]

Summary

• remove the fixed five-message cap from final, streaming, and mention delivery
• preserve full varied output using only platform-sized message splitting
• retain the narrow one/two-line degenerate repetition collapse
• add regression coverage proving outputs longer than five messages are not truncated

Validation

• rustfmt --edition 2021 --check src/adapter.rs
• cargo test --locked — 577 passed
• cargo build --release --locked

Run-Id: run-b44c4df0070cf3cd53b76eeb0ba72203
Trace-Id: trace-b34dc0e6d315716476d39616c3b75814
Root-Requester: Jun Discord id 473730953735438336
Implementer: MISA L / Linux MISA bot ID 1522164661028519996

[github-actions]

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