Add setting to control terminal resize overlay

[anthonykim1]

Resolves: #295790

• Add terminal.integrated.resizeDimensionsOverlay.enabled to control the terminal resize dimensions overlay.
• Keep the setting enabled by default to preserve existing behavior.
• Create or dispose the overlay when the setting changes.
• Apply setting changes to open terminals without requiring a reload or terminal restart.

Verification:

• Resize a terminal with the setting enabled and confirm the dimensions overlay appears.
• Disable the setting and confirm resizing no longer shows the overlay.
• Re-enable the setting and confirm the overlay returns without restarting the terminal.
• npm run typecheck-client
• npm run valid-layers-check

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Inspirations from:

• The setting schema follows the feature-owned terminal configuration pattern used by terminalCommandGuideConfiguration.
• Live setting updates and disposable feature activation follow TerminalCommandGuideContribution.
• The existing PTY-ready initialization delay is preserved from TerminalResizeDimensionsOverlayContribution.
• The setting is registered through the existing terminalContribConfiguration aggregation point.

[Copilot]

Pull request overview

Adds a configurable terminal resize-dimensions overlay with live updates for open terminals.

Changes:

• Registers terminal.integrated.resizeDimensionsOverlay.enabled, defaulting to enabled.
• Creates or disposes overlays when configuration changes.
• Integrates the setting into terminal configuration.

Show a summary per file

File	Description
terminalResizeDimensionsOverlayConfiguration.ts	Defines the new setting schema.
terminal.resizeDimensionsOverlay.contribution.ts	Applies the setting initially and on changes.
terminalContribExports.ts	Registers the setting with terminal configuration.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 1
• Review effort level: Medium