Reimplement Snakebids expand using SnakemakeFormatter

## Summary

Snakebids currently calls Snakemake’s `expand` directly. To support optional-wildcard templates and run cleanly in contexts where Snakemake may not be installed, we should implement Snakebids’ own `expand` behavior.

Key differences from Snakemake:

- For each template path, format it once per row of the zip-list using Snakebids’ `SnakemakeFormatter` (`snakebids/utils/snakemake_templates.py`).
- Snakebids does **not** allow a custom combinator function.
- Mixed-in additional entities are combined with the zip-list rows using **product**.
- Optional wildcards are supported.
- Output duplicates may arise when paths don’t contain all wildcards; we will do order-preserving deduplication via `dict.fromkeys(...)`.

Additionally:
- `allow_missing` must be supported, but implemented inside `SnakemakeFormatter`.
- If a wildcard is missing and `allow_missing=True`, the wildcard must remain **as the original brace-wrapped wildcard,constraint combination given as input**, i.e. the output should contain exactly the `{name,constraint}` form that appeared in the input template.

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### Context / links

- Discussion #486 (see GitHub UI)
- Discussion #491 (see GitHub UI)
- PR #494: https://github.com/khanlab/snakebids/pull/494
- PR #495: https://github.com/khanlab/snakebids/pull/495
- Template issues:
- #489: https://github.com/khanlab/snakebids/issues/489
- #508: https://github.com/khanlab/snakebids/issues/508

Upstream reference:
- Snakemake `expand`: `snakemake/snakemake` `src/snakemake/io/__init__.py`

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### Requirements

#### 1) Reimplement `expand` in Snakebids (stop calling Snakemake’s)
Modify Snakebids `expand` (in `src/snakebids/core/datasets.py`) to no longer call Snakemake’s `expand`. Instead, expansions should be generated by formatting template paths with `SnakemakeFormatter`.

#### 2) Formatting approach
For each provided template path:
- Iterate rows of the zip-list (zipped entities).
- For each row, format the path once using `SnakemakeFormatter.format(...)`.
- Mixed-in entities provided directly to `expand` should combine with zip-list rows using **product** (i.e., each zip row expands over all combinations of the mixed-in values).
- Snakebids `expand` does not support swapping the combinator function.

#### 3) Optional wildcards + missing entity handling
- Optional wildcards in templates are handled by `SnakemakeFormatter`.
- `None` and `""` both represent “missing entity” inputs; convert any `None` values to `""` prior to formatting.
- No dummy or special wildcard labels may be provided directly as entity arguments to `expand` (e.g. `_acq_`, `_subject_d_`, `__d__`, `___`). Ordinary entities may still be optional.

#### 4) `allow_missing` behavior is implemented in `SnakemakeFormatter`
- Add `allow_missing: bool = False` to `SnakemakeFormatter.__init__`.
- **When `allow_missing=True`, any missing wildcard must be replaced by the original brace-wrapped wildcard including constraint** as given in the input template, not by `""`.
- Constraint stripping and parsing must move from `parse` to `get_value`.

#### 5) Duplicate output handling
- Apply order-preserving deduplication: `list(dict.fromkeys(outputs))`.

#### 6) AnnotatedString flag preservation (Snakemake optional dependency)
- If an input template is an AnnotatedString with `.flags`, every output must be an AnnotatedString with those `.flags`.
- Snakebids must support running without Snakemake installed, gating this logic behind an import check.

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### Testing

Most required test scenarios are already covered in `tests/test_datasets.py` (notably `TestExpandables`). Additional tests should focus on:
- Ensuring `None` → `""` conversion for entity values before formatting with `SnakemakeFormatter`.
- Constraint-bearing wildcards are preserved verbatim under `allow_missing=True`.
- AnnotatedString propagation (conditional on snakemake installed).

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### Tasks

1. **Refactor `SnakemakeFormatter`**
- Add `allow_missing` to `__init__`.
- Stop stripping constraints in `parse()`.
- Implement constraint parsing/stripping logic inside `get_value`.
- When missing and `allow_missing=True`, return the original `{field_name}` verbatim (brace-wrapped, including constraint).

2. **Reimplement Snakebids `expand`**
- Replace direct Snakemake `expand` calls with formatter-based expansion.
- Ensure mixed-in entities are combined with zip-list rows using product.
- Convert `None` → `""` for entity values before formatting.
- Validate that no dummy/special wildcard labels are passed as direct entity args.
- Preserve order and deduplicate with `dict.fromkeys`.

3. **AnnotatedString flag propagation**
- Detect Snakemake `AnnotatedString` templates (only if snakemake installed).
- Ensure derived outputs are `AnnotatedString` with same `.flags`.

4. **Tests**
- Reuse existing dataset expansion tests in `tests/test_datasets.py` as primary coverage.
- Add focused tests for:
- Constraint-preserving partial expansion (`allow_missing=True`).
- `None` handling in expansion input values.
- AnnotatedString flag propagation (conditional).

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### Acceptance Criteria

- Snakebids no longer depends on calling Snakemake’s `expand`.
- `expand` uses `SnakemakeFormatter` for per-row formatting.
- `allow_missing` is implemented in `SnakemakeFormatter`, preserving brace-wrapped wildcards as specified.
- Optional entities support `None` and `""` as missing values, with correct conversion before formatting.
- Duplicate outputs are removed with order preserved.
- AnnotatedString flags are preserved when present and safely ignored if Snakemake is absent.
- All tests pass; code meets lint/typing standards.