refactor(core): remove legacy #[cfg(test)]-only dispatch harness duplicating production tool/LLM dispatch

[bug-ops]

Summary

• Removes the ~300-line #[cfg(test)]-only legacy sequential-dispatch harness in crates/zeph-core/src/agent/tool_execution/tool_result.rs (handle_tool_result, record_tool_output_outcome, process_successful_tool_output, handle_confirmation_required), superseded in production by process_one_tool_result/handle_confirmation_phase (tier_loop.rs)
• Removes the equivalent legacy LLM-dispatch harness in crates/zeph-core/src/agent/tool_execution/llm_dispatch.rs (call_llm_with_timeout, call_llm_non_streaming, call_llm_with_retry), superseded by call_chat_with_tools_retry/call_chat_with_tools/dispatch_chat_with_tools/call_non_streaming
• Repoints all ~20 dependent tests to exercise the real production entry points instead of the dead harnesses, so passing tests actually signal something about production behavior
• This class of duplication previously caused issue dump_tool_output (raw pre-summarization tool-output debug dump, #6315) is dead code — never called in production #6364 (a debug-dump call silently dead in production for months, masked by green legacy-harness tests) — removing the harness closes the gap at its root instead of leaving it to recur
• Corrects 4 stale doc references (2 crate READMEs, 2 mdbook pages) that described the removed functions as live production paths

Test plan

• cargo +nightly fmt --check
• cargo clippy --profile ci --workspace --all-targets --features "desktop,ide,server,chat,pdf,scheduler,testing" -- -D warnings
• cargo nextest run --config-file .github/nextest.toml --workspace --features "desktop,ide,server,chat,pdf,scheduler" --lib --bins — 15006 passed, 0 failed, 36 skipped
• Rustdoc gate (RUSTFLAGS="-D warnings" RUSTDOCFLAGS="--deny rustdoc::broken_intra_doc_links")
• gitleaks protect --staged
• Independent review re-ran the full check suite and verified zero production-line changes in either touched file (100% of removed lines were pre-existing #[cfg(test)]-gated code)

Closes #6560