DeclarativeExecutor passing down empty configs

## Steps to reproduce

```
import airbyte as ab
source = ab.get_source(
"example-test",
config={"hello": "world"},
source_manifest=Path(os.getcwd()) / "mymanifest.yaml",
streams="*"
)
source.check()
```

### Effects
The YAML is built with {{ config['hello'] }} as an empty string.

## Root cause

https://github.com/airbytehq/PyAirbyte/blob/7eb746bda4a5e72b4643b6af7692d6a736080e60/airbyte/_executors/declarative.py#L125

`self.declarative_source` is accessing `self._config_dict` that is always empty, apart from `components_py`. Since the arg parsing is performed later in the `execute()` before calling the `source_entrypoint.run(args)`

```
def declarative_source(self) -> ConcurrentDeclarativeSource:
"""Get the declarative source object.

Notes:
1. Since Sep 2025, the declarative source class used is `ConcurrentDeclarativeSource`.
2. The `ConcurrentDeclarativeSource` object sometimes doesn't want to be read from twice,
likely due to threads being already shut down after a successful read.
3. Rather than cache the source object, we recreate it each time we need it, to
avoid any issues with re-using the same object.
"""
return ConcurrentDeclarativeSource(
config=self._config_dict,
source_config=self._manifest_dict,
)
```

## Proposed fix
Promote config_dict in the initialization params of DeclarativeExecutor and use it instead of initializing it as an empty dict.