Fix VS Code extension strict comparison lint warnings

[Copilot]

Description

Fixes the remaining VS Code extension lint warnings caused by non-strict != null comparisons in the App Host tree provider, code lens provider, and gutter decoration provider.

This keeps the existing exit-code behavior while aligning the checks with the extension lint rules.

Validation:

• parallel_validation (Code Review + CodeQL) ✅
• npm run lint could not be executed in this sandbox because npm ci failed on blocked network access while downloading packages

cc @adamint

Fixes # (issue)

Checklist

• Is this feature complete?
  • Yes. Ready to ship.
  • No. Follow-up changes expected.
• Are you including unit tests for the changes and scenario tests if relevant?
  • Yes
  • No
• Did you add public API?
  • Yes
    • If yes, did you have an API Review for it?
      • Yes
      • No
    • Did you add <remarks /> and <code /> elements on your triple slash comments?
      • Yes
      • No
  • No
• Does the change make any security assumptions or guarantees?
  • Yes
    • If yes, have you done a threat model and had a security review?
      • Yes
      • No
  • No
• Does the change require an update in our Aspire docs?
  • Yes
    • Link to aspire.dev issue:
      • New issue
  • No

[github-actions]

🚀 Dogfood this PR with:

⚠️ WARNING: Do not do this without first carefully reviewing the code of this PR to satisfy yourself it is safe.

curl -fsSL https://raw.githubusercontent.com/microsoft/aspire/main/eng/scripts/get-aspire-cli-pr.sh | bash -s -- 15837

Or

• Run remotely in PowerShell:

iex "& { $(irm https://raw.githubusercontent.com/microsoft/aspire/main/eng/scripts/get-aspire-cli-pr.ps1) } 15837"

[adamint]

all these changes are fine, i just wonder if it might be better to add a type guard and avoid all the duplication?

ie,

function isNotNullOrUndefined<T>(val: T | undefined | null): val is T {
    return val !== undefined && val !== null;
}