Implement action for branch picker with uncommitted files

[Ladislau Szomoru (lszomoru)]

This pull request implements an action for the branch picker in the agents window that is displayed only when the current branch has uncommitted files. The action allows users to reveal the auxiliary bar and focus on the Changes view.

Key changes include:

• Added a Show Changes action for dirty checked-out branch rows.
• Clean and non-current branch rows do not display the action.
• Fixed hover layout shift by reserving space for the action toolbar.
• Resolved focus issues to ensure the Changes tab is selected when the action is triggered.
• Added regression tests to cover the new functionality and layout changes.

Validation: All relevant tests passed, including layout and regression tests.

[Copilot]

Copilot review overview

Review tier: Balanced
Findings: 1

New issues introduced by this change (1)

Severity	Finding
	src/​vs/​platform/​actionWidget/​browser/​actionWidget.css — visibility: hidden keeps this flex toolbar in layout, so it is already included in the row's…

What changed in this PR

Adds a dirty-branch action that opens the session’s Changes UI while stabilizing picker-row layout.

Changes:

• Adds Show Changes to dirty checked-out branches.
• Opens the Changes editor/view in the correct order.
• Adds action and layout regression tests.

File	Description
agentHostSessionConfigPicker.ts	Adds and handles the branch action.
agentHostSessionConfigPicker.test.ts	Tests visibility and navigation.
actionList.ts	Marks rows containing detail text.
actionWidget.css	Reserves toolbar space.
actionList.test.ts	Tests stable toolbar geometry.

Suppressed comments (1)

src/vs/platform/actionWidget/browser/actionWidget.css:471

• This computes to 10px, but the new regression test asserts toolbarMarginRight === '6px' in both states, so the test will deterministically fail with this stylesheet loaded. Align the implementation with the asserted geometry.

	margin-right: 10px;

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