Fix kit source and prefix validation when sharing source directory

[shettyvarun268]

Description

Fixes an issue where installing a local Function Kit into a directory that is already configured as a functions codebase in firebase.json (for example, firebase functions:kits:install --directory functions) causes subsequent firebase commands to fail validation with:

More than one functions config specifies the same source directory ('functions') and prefix ('').

In assertUniqueSourcePrefixPair (src/functions/projectConfig.ts), kit configs (isKitConfig(c)) previously defaulted to c.prefix || "" ("") even though kit configurations do not allow a top-level prefix field and instead automatically prefix each instance with addKitPrefix(instanceId) (kit-<instanceId>) during config expansion.

This change:

• Updates assertUniqueSourcePrefixPair to resolve kit prefixes per instance via addKitPrefix(instanceId) (kit-<instanceId>).
• Runs assertUniqueKitInstancesAndCodebases before assertUniqueSourcePrefixPair in validate() so duplicate kit instance IDs across kits sharing a source directory continue to report the duplicate-instance error first.

Scenarios Tested

• Unit tests in src/functions/projectConfig.spec.ts:
  • A local codebase ({ source: "functions", codebase: "default" }) and a kit config sharing source: "functions" pass validation.
  • Multiple kit configs with distinct instance IDs sharing the same source directory pass validation.
  • A local codebase with an explicit prefix that matches a kit instance's resolved prefix (kit-<instanceId>) on the same source directory fails validation.

Sample Commands

firebase functions:kits:install --directory functions

[gemini-code-assist]

Code Review

This pull request resolves an issue where a Function Kit and a functions codebase sharing the same source directory failed firebase.json validation. It updates the validation logic in assertUniqueSourcePrefixPair to correctly extract and check prefixes for kit instances, and adds comprehensive unit tests. The review feedback points out that the error message in assertUniqueSourcePrefixPair should be updated to provide accurate remediation guidance, since specifying a prefix is forbidden in a Function Kit configuration.