fix(filesystem): default bucket_url to '.' for bare destination string

[Bhagwat-45]

Description

Allows bare dlt.pipeline(..., destination="filesystem") without requiring bucket_url to be set in config.toml, matching the existing behavior of the duckdb destination.

Previously this would throw a ConfigFieldMissingException. The fix sets bucket_url = "." (current directory) as the default in FilesystemConfiguration. The existing normalize_bucket_url() method already handles "." correctly, converting it to an absolute file:// URL at resolve time — so this is a minimal, safe one-line change.

Files changed:

• dlt/common/storages/configuration.py — set bucket_url: str = "." as the default
• tests/sources/filesystem/test_config_sections.py — rewrote test to reflect new default; added test verifying config section path for cloud destinations (which still require credentials)
• tests/load/filesystem/test_filesystem_factory.py — updated stale comment
• tests/workspace/cli/test_config_toml_writer.py — updated assertion and comment

Related Issues

• Fixes feat(filesystem): allow bare dlt.pipeline(..., destination="filesystem") #3886

Additional Context

Tests were run locally and passed:

pytest tests/common/storages/test_local_filesystem.py \
       tests/sources/filesystem/test_config_sections.py \
       tests/load/filesystem/test_filesystem_factory.py \
       tests/workspace/cli/test_config_toml_writer.py -v

Result: 55 passed, 21 skipped, 0 failed.

[rudolfix]

@Bhagwat-45 by setting a default you are making bucket_url optional. this will impact a lot of users on filesystems different than local. same configuration is shared with ducklake, lance and iceberg catalogs. tbh. it will be very confusing if instead of failing with missing configuration, files are created in locally. very hard to diagnose and also leaks data into place where it should not be. my take is that you add the following to your global config toml (so all your projects use it)

[credentials]
bucket_url="file:///."

(I didnt check it, pwd may be slightly different)
this will act as an default that any more specific configuration can overwrite

[zilto]

@rudolfix the initial issue was for quality of life and supporting, similar to destination="duckdb"

dlt.pipeline(..., destination="filesystem")

Setting up config.toml or dlt.destinations.filesystem(bucket_url=...) can trip up new users and agents

Potential solutions

• IMHO, it makes sense to have filesystem default to local. It doesn't make sense to expect your pipeline will write to S3 if you didn't set any credentials
• The tricky part is how filesystem credentials are used by other destinations. In these cases, can the "parent config" set default value to None / missing? For example, ducklake could remove bucket_url="."

If this is too complicated, we should avoid changes.