Profile config files silently ignored without .dlt/.workspace marker

Problem

When profile-scoped config files like dev.config.toml or prod.config.toml exist in .dlt/, they are silently ignored unless a .dlt/.workspace marker file is present. There is no warning or error indicating that these files are not being loaded.

This leads to confusing behavior where:

1. You split destination config into dev.config.toml and prod.config.toml (e.g., different bucket_url per environment)
2. Remove the shared value from config.toml to avoid conflicts
3. Pipeline fails with ConfigFieldMissingException for bucket_url
4. The error message lists all providers searched but gives no hint that profile config files exist on disk but are not being loaded

Root cause

Without .dlt/.workspace, the run context falls back to plain RunContext instead of WorkspaceRunContext. The profile-scoped TOML providers (ProfileSecretsTomlProvider, etc.) are never registered, so dev.config.toml / prod.config.toml are never read -- even though they're sitting right there in .dlt/.

Expected behavior

At minimum, dlt should warn when .dlt/{profile}.config.toml or .dlt/{profile}.secrets.toml files exist but profile loading is inactive because .dlt/.workspace is missing. Something like:

WARNING: Found profile config files (dev.config.toml, prod.config.toml) in .dlt/ 
but workspace is not initialized. These files will not be loaded. 
Create .dlt/.workspace to enable profile support.

Ideally, the ConfigFieldMissingException output should mention the unloaded profile files when they exist on disk and could contain the missing value.

Steps to reproduce

# Create profile configs
cat > .dlt/dev.config.toml << 'TOML'
[destination.filesystem]
bucket_url = "./out/local-data"
TOML

cat > .dlt/prod.config.toml << 'TOML'
[destination.filesystem]
bucket_url = "s3://my-bucket"
TOML

# Remove bucket_url from shared config (since it's now per-profile)
# .dlt/config.toml has no [destination.filesystem] section

# Do NOT create .dlt/.workspace

# Run pipeline -- fails with ConfigFieldMissingException for bucket_url
uv run python my_pipeline.py

Fix: touch .dlt/.workspace -- then profile configs load correctly.

Related

• #3788 -- dlt profile command should always be available with guidance when workspace is not initialized (discoverability side of the same problem)

Environment

• dlt version: 1.24.0
• dlthub version: 0.22.1
• Python: 3.13
• OS: macOS

Trace how RunContext and WorkspaceRunContext register ProfileSecretsTomlProvider and related profile-scoped providers when .dlt/.workspace is absent. Reproduce the missing bucket_url case, then determine where a warning or ConfigFieldMissingException hint can be added; done means existing profile files are surfaced without changing behavior when workspace support is enabled.