Fix interactive terminal prompt scroll locking

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Description

Fixes incorrect scroll locking for non–alt-screen interactive long-running prompts.

Problem: Viewport scrolling followed the bottom of the backing grid instead of the live PTY cursor, so users could scroll into empty grid space below the active prompt during flows like interactive CLIs.

Solution: Clamp scroll position and scrollbar range to the interactive cursor boundary, add scroll modes that preserve that boundary when the user scrolls manually, and apply the interactive follow update path on terminal wakeup when appropriate so prompt output stays visible without requiring a keystroke first.

Touches: app/src/terminal/block_list_viewport.rs, app/src/terminal/block_list_element.rs, app/src/terminal/view.rs; tests in app/src/terminal/view_test.rs.

Linked Issue

Fixes #9019

• The linked issue is labeled ready-to-spec or ready-to-implement.
• Where appropriate, screenshots or a short video of the implementation are included below (especially for user-visible or UI changes).

Video (Before and after)

scroll.issue.fixed.mp4

Testing

• New/updated unit tests in app/src/terminal/view_test.rs for interactive long-running scroll follow, manual scroll-away, and related viewport behavior.

Agent Mode

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

Changelog Entries for Stable

CHANGELOG-BUG-FIX: Fixed interactive long-running prompts scrolling past the live cursor into empty backing-grid space.

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Overview

This PR adds an interactive-cursor scroll mode for long-running terminal prompts, clamps the scroll range to that boundary, updates PTY input/wakeup paths to use it, and adds viewport tests.

Concerns

• Waterfall gap follow state is treated as a fixed/manual state during interactive PTY input, so Waterfall mode can keep the old bottom-of-backing-grid scroll range instead of switching to the live cursor boundary.

Verdict

Found: 0 critical, 1 important, 0 suggestions

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⚠️ [IMPORTANT] Waterfall mode can still miss the interactive cursor boundary here. WaterfallGapFollowsBottomOfMostRecentBlock is a follow state, not a manual scroll-away state, so returning it unchanged leaves effective_max_scroll_top_in_lines() on the full backing-grid max and re-exposes the blank area this PR is trying to clamp.

Suggested change

	ScrollPosition::FollowsBottomOfMostRecentBlock
	| ScrollPosition::FollowsInteractiveCursor => ScrollPosition::FollowsInteractiveCursor,
	ScrollPosition::WaterfallGapFollowsBottomOfMostRecentBlock { .. }
	ScrollPosition::FollowsBottomOfMostRecentBlock
	| ScrollPosition::FollowsInteractiveCursor
	| ScrollPosition::WaterfallGapFollowsBottomOfMostRecentBlock { .. } => {
	ScrollPosition::FollowsInteractiveCursor
	}
	ScrollPosition::FixedAtPosition { .. }
	| ScrollPosition::FixedAtInteractivePosition { .. }
	| ScrollPosition::FixedWithinLongRunningBlock { .. }
	| ScrollPosition::FixedWithinInteractiveLongRunningBlock { .. } => self.scroll_position,

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[oz-for-oss]

Overview

This PR adds interactive-cursor scroll positions, clamps scrollbar range to the live cursor for long-running prompts, and adds unit coverage for pinned-to-bottom and pinned-to-top cursor-follow behavior.

Concerns

• Pinned-to-top sessions can enter FollowsInteractiveCursor, but the new manual-scroll boundary preservation only applies to pinned-to-bottom and waterfall modes, so manually scrolling in pinned-to-top can still fall back to the full backing-grid range.

Verdict

Found: 0 critical, 1 important, 0 suggestions

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⚠️ [IMPORTANT] PinnedToTop can enter FollowsInteractiveCursor, but this excludes it from preserving the cursor boundary on manual scrolls; a wheel scroll downgrades to non-interactive fixed modes and re-enables the backing-grid bottom. Include PinnedToTop here or avoid entering interactive follow mode for it.