fix(compiler): improve error message when --emit specifies an uninstalled emitter

[Copilot]

tsp compile . --emit not-installed-emitter previously emitted a generic import-not-found error with no actionable guidance. It now reports a dedicated emitter-not-found diagnostic with an install hint.

Before:

error import-not-found: Couldn't resolve import "not-installed-emitter"

After:

error emitter-not-found: Emitter "not-installed-emitter" not found. Make sure to install it with `npm install not-installed-emitter`.

Changes

• messages.ts: New emitter-not-found diagnostic code with a message that names the emitter and includes an npm install hint.
• program.ts: loadLibrary accepts an optional { isEmitter } context flag; when set and the underlying error is import-not-found, it reports emitter-not-found instead. loadEmitter passes { isEmitter: true }.

• Fixes --emit invalid-emitter doesn't give the best error message #1982

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[pkg-pr-new]

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npm i https://pkg.pr.new/@typespec/compiler@11485

npm i https://pkg.pr.new/@typespec/playground@11485

commit: 954f504

[azure-sdk-automation]

You can try these changes here

🛝 Playground	🌐 Website	🛝 VSCode Extension

[timotheeguerin]

@copilot pull main and resolve conflicts

[Copilot]

@copilot pull main and resolve conflicts

Done — merged main and resolved the conflict in emitter.test.ts (combined the createTypeSpecLibrary import from main with the expectDiagnostics import from our branch). All tests pass.

[timotheeguerin]

@copilot one test is failing now ❯ test/deferred-emitter-compile.test.ts:109:51

[Copilot]

@copilot one test is failing now ❯ test/deferred-emitter-compile.test.ts:109:51

Fixed — updated packages/playground/test/deferred-emitter-compile.test.ts line 109 to expect emitter-not-found instead of import-not-found. Committed in the latest push.