Fix Add Agent dialog content overflow

[Tryanks]

The Add Agent dialog painted its catalog and Custom Agent action below the modal because the panel used its natural height and the catalog reserved a separate 360px viewport. Propagate the dialog body height through the panel and let the catalog scroll in the remaining space. Use the shared dialog height cap instead of subtracting another fixed window allowance, which also keeps the action inside a 360×480 window.

Validation:

• Added a GPUI regression test through the real Settings → Add Agent dialog at 1000×780, 393×600 and 360×480. Before the fix, the custom action ended at 764.5px while the body ended at 596px; the final test passes.
• cargo fmt --all --check
• cargo clippy --workspace --all-targets --locked -- -D warnings
• cargo nextest run --workspace --locked: 914 passed, 6 skipped.
• cargo-machete .: no unused dependencies (the local Cargo subcommand shim incorrectly treats machete as a directory).
• Launched the macOS app with a disposable profile and checked light/dark themes at wide and narrow widths, including scrolling the populated catalog to its last entry. Temporary geometry/theme instrumentation was removed before commit.
• Other desktop platforms, mobile and Web are covered by PR CI.