Code mappings: Support multi-project repositories with shared path structures

### Problem Statement

Code mappings have a unique constraint on (project, stack_root), which prevents configuring multiple source folders for the same stack trace path pattern.

This creates a limitation for multi-project repositories (common in .NET solutions) where:
- Multiple projects live in different subfolders (e.g., ProjectA/, ProjectB/)
- Stack traces show relative paths without project identifiers (e.g., Services/Foo.cs)
- The same relative path structure exists across multiple project folders

Users cannot get stack trace linking to work for all projects in their solution - only one folder can be mapped per stack trace root.

See linear ticket for more info.

### Solution Brainstorm

Allow multiple source folders to be checked for the same stack trace root pattern, trying each until a matching file is found.

### Product Area

Settings - Integrations

Start by locating the code-mapping implementation and the unique constraint on (project, stack_root). Review how stack-trace paths are matched, then verify that multiple source folders can be configured for one stack-trace root and are tried until a matching file is found.

Written by the indexing model from the issue text.