fix: migrate from deprecated tseslint.config to eslint/config

[JoshuaKGoldberg]

Description

Switches from the deprecated tseslint.config() to ESLint core's defineConfig(). I also threw in some little touchups here and there for fun, but I can split those out if you prefer. 😄

Fixes #15408.

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 (existing tests)
  • 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

I know #15408 hasn't been triaged yet, but figured this was a straightforward change 🙂 apologies if I've broken some unspoken etiquette rule.

[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/dotnet/aspire/main/eng/scripts/get-aspire-cli-pr.sh | bash -s -- 15409

Or

• Run remotely in PowerShell:

iex "& { $(irm https://raw.githubusercontent.com/dotnet/aspire/main/eng/scripts/get-aspire-cli-pr.ps1) } 15409"

[JoshuaKGoldberg]

[Explanation] As part of the move to eslint/config's helpers, we've seen users have a much easier time understanding configs with globalIgnores vs. the "object that just has ignores" equivalent. The two do the same thing.

[JoshuaKGoldberg]

...

[Explanation] This ... is unnecessary.

[JoshuaKGoldberg]

[Explanation] In ESLint flat config, ecmaVersion defaults to "latest". Which is great in particular for TypeScript users: TypeScript will already catch any too-new-for-the-target-runtime syntax. There's no need to explicitly go back five ecmaVersions.

[JoshuaKGoldberg]

[Explanation] This is inferred from the ESLint config path now! A relatively recent quality-of-life improvement / simplification on the typescript-eslint side: typescript-eslint/typescript-eslint#11370

[JoshuaKGoldberg]

@dotnet-policy-service agree

[Copilot]

Pull request overview

Updates Aspire’s TypeScript/React template ESLint flat configs to use ESLint core’s defineConfig() (via eslint/config) instead of the deprecated tseslint.config() helper, aligning templates with current ESLint guidance (fixes #15408).

Changes:

• Migrated multiple eslint.config.* files from tseslint.config() to defineConfig() and introduced globalIgnores() for global ignore patterns.
• Removed tsconfigRootDir: import.meta.dirname from TypeScript AppHost ESLint configs used by scaffolding/templates.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/Aspire.ProjectTemplates/templates/aspire-ts-cs-starter/frontend/eslint.config.js	Switches to defineConfig() + globalIgnores() for the TS/React starter template.
src/Aspire.ProjectTemplates/templates/aspire-py-starter/frontend/eslint.config.js	Switches to defineConfig() + globalIgnores() for the Python starter’s frontend template.
src/Aspire.Hosting.CodeGeneration.TypeScript/TypeScriptLanguageSupport.cs	Updates scaffolded TypeScript AppHost ESLint config by removing tsconfigRootDir.
src/Aspire.Cli/Templating/Templates/ts-starter/frontend/eslint.config.js	Switches the CLI TS starter frontend ESLint config to defineConfig() + globalIgnores().
src/Aspire.Cli/Templating/Templates/ts-starter/eslint.config.mjs	Removes tsconfigRootDir from the TS AppHost ESLint config template.
playground/FoundryAgentEnterprise/frontend/eslint.config.js	Switches playground frontend ESLint config to defineConfig() + globalIgnores().