fix(cli): error when --instance does not match any configured docs instance

[devin-ai-integration]

Description

When running fern generate --docs --instance <url> with a URL that doesn't exactly match any configured instance in docs.yml, the CLI silently falls back to publishing to instances[0] instead of erroring.

This caused docs to be published to the wrong production site — the user specified a Glean instance URL but docs were published to the first instance (partner-docs-ddn).

Changes Made

In runRemoteGenerationForDocsWorkspace.ts:

- const maybeInstance = instances.find((...) => ...) ?? instances[0];
+ const maybeInstance =
+     instanceUrl != null ? instances.find((...) => ...) : instances[0];

When instanceUrl is provided:

• Only use find() — no fallback to instances[0]
• If no match, failAndThrow with a message listing available instances

When instanceUrl is undefined (no --instance flag):

• Preserve existing behavior: use instances[0]

The CLI-v2 PublishCommand.resolveInstanceUrl already handles this correctly by throwing CliError on mismatch; this fix aligns the v1 code path.

Testing

• pnpm run check passes
• Manual review of v2 path confirms it already has the correct behavior

Link to Devin session: https://app.devin.ai/sessions/4d76819a21d049f7a33174acd031e595

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[devin-ai-integration]

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.