fix(channels): forward elicit() and send_context_estimate() on AppChannel

[bug-ops]

Summary

• impl Channel for AppChannel (src/channel.rs) forwarded most Channel trait methods via dispatch_app_channel! but never overrode elicit() or send_context_estimate(), so both calls silently fell through to the trait's default methods instead of reaching CliChannel/TuiChannel's real, already-unit-tested implementations.
• elicit()'s default unconditionally returns Declined, so every MCP server-driven elicitation request was auto-declined in both CLI and TUI modes regardless of [mcp] elicitation_enabled. send_context_estimate()'s default silently no-ops, so the TUI's real-time context-usage estimate never reached the input block title.
• Added both missing overrides, following the existing dispatch_app_channel! pattern used by the other forwarded methods.
• Live-verified against a real stdio MCP fixture server: the [MCP server '...' is requesting input] banner and the real per-field prompt now fire, where previously the request was auto-declined in under 1ms with no visible indication.

Note: this fix makes elicitation prompts reachable again, but a separate, pre-existing bug (#6398, filed during this fix's live verification) means the CLI accept path can race against the background chat-input reader for stdin and time out client-side — that is a distinct root cause, not caused by this change, and is tracked separately.

Closes #6388

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 — 14007 passed, 0 failed, 35 skipped
• RUSTFLAGS="-D warnings" RUSTDOCFLAGS="--deny rustdoc::broken_intra_doc_links" cargo doc --no-deps --workspace --features "desktop,ide,server,chat,pdf,scheduler"
• RUSTFLAGS="-D warnings" cargo check --workspace --all-targets --features desktop,ide,server,chat,pdf,scheduler --locked
• cargo test --doc --workspace --features "desktop,ide,server,chat,pdf,scheduler"
• gitleaks protect --staged --no-banner --redact — no leaks
• New regression tests added for both overrides (app_channel_forwards_elicit_to_real_implementation, app_channel_forwards_send_context_estimate_to_real_implementation), each independently revert-verified to fail (not pass vacuously) when its override is removed
• Live e2e verification via tmux against a real stdio MCP fixture server confirms the CLI elicitation prompt now fires