Fix MCP JSON viewer key vertically centered against tall expanded string

[warp-agent-staging]

Description

Fixes the MCP JSON viewer key alignment bug reported in Slack: when the request/response JSON tree for an MCP tool call is expanded and a key's string value is expanded and wraps to many lines (e.g. a long multi-line note field), the key label and chevron were vertically centered against the whole tall value block instead of sitting on the value's first line.

Root cause: render_long_string_row in app/src/ui_components/json_tree.rs put the chevron, key label, and value into a single Flex::row() with CrossAxisAlignment::Center. That's correct while the value is a single line, but once the expanded string wraps to multiple lines, Center centers the chevron/key against the entire value block instead of its first line.

Linked Issue

Fixes APP-5312.

• Screenshots included below.

Changes

• render_long_string_row: group the chevron and key label into their own nested Flex::row() with CrossAxisAlignment::Center (so they stay centered against each other, as before), and add that group as a single child of the outer row, which now uses CrossAxisAlignment::Start unconditionally. The chevron+key pair's line box top-aligns with the value's first line, whatever the value's height — so there's no longer a branch on expansion state, and the row looks the same as before for single-line (collapsed/short) values.
• render_container_node and render_scalar_row were left untouched — their rows only ever render a single line of content, so they can't exhibit this bug, and their output is unaffected by this change.

Testing

• cargo build -p warp --bin warp — succeeds.
• cargo clippy -p warp --all-targets --tests -- -D warnings — passes with no new warnings.
• cargo nextest run -p warp --lib json_tree — all 34 existing json_tree unit tests pass unchanged.
• Per the requester's instruction, no new tests or comments were added.

Screenshots / Videos

Verified by driving the actual render_json_tree / render_long_string_row production code path (from app/src/ui_components/json_tree.rs) in a temporary, uncommitted scratch example, with a payload shaped like the report: an object with a few short keys (id, status) and one long note value that wraps to ~30 lines, rendered both collapsed and expanded.

Full window — both sections:

Close-up of the expanded note row — the chevron is centered against the note: label as a pair, and that pair sits on the value's first line:

Observed: the id/status single-line rows and the collapsed note row (single-line truncated preview) are pixel-unaffected and identical across both sections — only the expanded, multi-line case changes.

• I have manually tested my changes locally with ./script/run

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

CHANGELOG-BUG-FIX: Fixed the MCP tool-call JSON viewer's key/chevron being vertically centered against a tall expanded multi-line string value instead of aligned to its top.

Conversation: https://staging.warp.dev/conversation/2b5f2fc5-0a58-4b64-ae97-899f9985870b
Run: https://oz.staging.warp.dev/runs/019ff1e8-1dd5-707a-a1f8-46dd41a7c505

This PR was generated with Oz.