sessions: make navigation history size observable

[sandy081]

Follow-up to #315382.

Addresses two review comments from Copilot:

1. _history.length is not observable — _canGoForward derived read this._history.length (a plain array), so mutations that don't change _currentIndex (e.g. session removal at a position after the current index) could leave the derived stale. Fix: add a _historySize observable, update it in all mutation paths (_pushEntry, _navigateTo, _removeEntriesMatching), and read it in the _canGoForward derived.

2. Misleading comment — _navigateTo said "remove it and try again" but the code only removes the missing entry without retrying. Updated the comment to match the actual behavior.

[Copilot]

Pull request overview

This PR follows up on the Agents window session navigation history implementation by making the history size observable (so canGoForward updates correctly when the backing array changes) and by correcting an inaccurate comment in the missing-session navigation path.

Changes:

• Introduces _historySize as an observable and uses it in the _canGoForward derived.
• Updates all history mutation paths to keep _historySize in sync.
• Adjusts a comment in _navigateTo to match the actual behavior when a session no longer exists.

Show a summary per file

File	Description
src/vs/sessions/services/sessions/browser/sessionNavigation.ts	Adds an observable for history length to keep forward-navigation context state correct and updates comments around missing-session handling.

Copilot's findings

Comments suppressed due to low confidence (1)

src/vs/sessions/services/sessions/browser/sessionNavigation.ts:220

• _removeEntriesMatching can set _currentIndex to 0 even when all history entries are removed (because newCurrentIdx can become -1 but is clamped via Math.max(0, ...)). This makes the index point to a non-existent entry and can incorrectly keep Back enabled if _beyondHistory is true. Handle the empty-history case explicitly (e.g. set _currentIndex to -1 and clear _beyondHistory when this._history.length === 0).

		if (newCurrentIdx !== currentIdx) {
			this._currentIndex.set(Math.max(0, newCurrentIdx), undefined);
		}
		this._historySize.set(this._history.length, undefined);
	}

• Files reviewed: 1/1 changed files
• Comments generated: 1

[github-actions]

blocks-ci screenshots changed

Replace the contents of test/componentFixtures/blocks-ci-screenshots.md with:

Updated blocks-ci-screenshots.md

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