[APP-4261] Add tooltips to disabled git operation buttons (#9234)

## Description

Adds contextual tooltips to disabled git operation buttons so users
understand why they can't click them. Tooltips are intentionally limited
to **disabled states only** — enabled buttons don't get tooltips since
their labels are self-explanatory.

**Changes:**
- Code review header: "No changes to commit" on the disabled Commit
button; "No git actions available" on the disabled chevron
- Commit dialog: "Enter a commit message" on the disabled Confirm button
(only shown once file changes have loaded — silent during the async load
window)
- Bug fix: the PR #N button in the header was staying greyed out after
transitioning from a disabled Commit mode, because `ViewPr` mode never
called `set_disabled(false)`

Linear:
https://linear.app/warpdotdev/issue/APP-4261/git-buttons-should-all-have-tooltips

Warp conversation:
https://staging.warp.dev/conversation/27f5811f-22ea-4c4b-8d8f-47ff48a5b12b

Loom: https://www.loom.com/share/932cfb22039c4566bcc7a7343b624d81

## Testing

Manually verified:
- Commit button with no changes shows "No changes to commit" on hover
- Chevron with no changes shows "No git actions available" on hover
- Commit dialog Confirm button shows "Enter a commit message" once files
load but message is empty; no tooltip shown during async load
- PR #N button is no longer greyed out after committing all changes

No new automated tests added — these are tooltip strings on existing
button state logic that is already exercised by existing tests.

## Server API dependencies

N/A — client-only change.

## Agent Mode
- [x] Warp Agent Mode - This PR was created via Warp's AI Agent Mode

## Changelog Entries for Stable

CHANGELOG-IMPROVEMENT: Git operation buttons now show tooltips
explaining why they're disabled (e.g. "No changes to commit", "Enter a
commit message").

Co-Authored-By: Oz <oz-agent@warp.dev>

---------

Co-authored-by: Oz <oz-agent@warp.dev>

app/src/code_review/code_review_view.rs

@@ -6798,11 +6798,12 @@ impl CodeReviewView {
 
         match mode {
             PrimaryGitActionMode::Commit => {
-                let has_changes = self.has_uncommitted_changes(ctx);
+                let disabled = !self.has_uncommitted_changes(ctx);
                 self.git_primary_action_button.update(ctx, |button, ctx| {
                     button.set_label("Commit", ctx);
                     button.set_icon(Some(Icon::GitCommit), ctx);
-                    button.set_disabled(!has_changes, ctx);
+                    button.set_disabled(disabled, ctx);
+                    button.set_tooltip(disabled.then_some("No changes to commit"), ctx);
                     button.set_on_click(
                         |ctx| ctx.dispatch_typed_action(CodeReviewAction::OpenCommitDialog),
                         ctx,
@@ -6810,7 +6811,8 @@ impl CodeReviewView {
                     button.set_adjoined_side(AdjoinedSide::Right, ctx);
                 });
                 self.git_operations_chevron.update(ctx, |button, ctx| {
-                    button.set_disabled(!has_changes, ctx);
+                    button.set_disabled(disabled, ctx);
+                    button.set_tooltip(disabled.then_some("No git actions available"), ctx);
                 });
             }
             PrimaryGitActionMode::Push => {
@@ -6849,6 +6851,7 @@ impl CodeReviewView {
                     self.git_primary_action_button.update(ctx, |button, ctx| {
                         button.set_label(label, ctx);
                         button.set_icon(Some(Icon::Github), ctx);
+                        button.set_disabled(false, ctx);
                         button.set_on_click(
                             move |ctx| {
                                 ctx.dispatch_typed_action(CodeReviewAction::ViewPr(url.clone()))