Alembic autogenerate fails with KeyError: 'name' due to missing constraint metadata

#### AI generated bug ticket (but confirmed by me)

When running **`alembic revision --autogenerate`** against a BigQuery‑backed SQLAlchemy metadata model, the autogenerate process **crashes with**:

KeyError: 'name'

The error originates inside **Alembic’s constraint‑reflection normalization** (`_apply_reflectinfo_conv`), which unconditionally accesses `const["name"]`. The BigQuery dialect returns constraint dictionaries **without a `'name'` field**, causing Alembic to fail during autogenerate.

This makes Alembic migrations unusable for any project that uses SQLAlchemy + BigQuery + autogenerate.

***

## **Environment**

* **Python:** 3.11
* **SQLAlchemy:** 2.0.46
* **Alembic:** 1.18.2
* **sqlalchemy-bigquery:** 1.16.0
* **Backend:** Google BigQuery
* **ORM style:** Imperative mapping with polymorphic inheritance

***

## **Error details**

### **Traceback excerpt**

File ".../alembic/autogenerate/compare/util.py", line 259, in _apply_reflectinfo_conv
if const["name"] is not None and not isinstance(
KeyError: 'name'

### **What triggers it**

* Alembic reflects constraints (PKs, UNIQUE, FKs, CHECK, etc.)
* BigQuery dialect’s implementation of:
* `Inspector.get_pk_constraint()`
* `Inspector.get_unique_constraints()`
* (and potentially others)

returns dictionaries that **omit mandatory keys**, especially `'name'`.

Alembic expects at minimum:

```python
{
"name": ,
"constrained_columns": [...],
}
```

but BigQuery returns incomplete structures.

***

## **Related existing issue(s)**

This appears closely related to (but not identical to):

* **#866 — “sqlalchemy inspect constraints support”**
That issue documents that **constraint reflection is not implemented**, and `get_pk_constraint()` / `get_foreign_keys()` return empty or incomplete structures.
This missing data directly causes Alembic to crash during autogenerate.

However, **there is currently no issue specifically tracking Alembic autogenerate failures caused by missing `'name'` in reflected constraint dictionaries**, so filing this as a distinct bug.

***

## **Expected behavior**

Alembic autogenerate should:

* Successfully compare ORM metadata vs. reflected DB state
* Not crash due to missing fields
* Treat missing constraint information as either:
* empty constraints
* or unsupported constraint types

The dialect should implement constraint reflection methods that meet SQLAlchemy’s expected return formats.

***

## **Actual behavior**

* Constraint dictionaries returned by the BigQuery dialect **lack required fields**, e.g. `'name'`
* Alembic crashes during its normalization phase
* `--autogenerate` cannot be used at all for BigQuery projects

***

## **Minimal reproduction**

1. Define any model with a primary key in SQLAlchemy.
2. Point engine at BigQuery using `sqlalchemy-bigquery`.
3. Run:
```bash
alembic revision --autogenerate -m "test"
```
4. Autogenerate fails with `KeyError: 'name'`.

No foreign keys or unique constraints are required — even a simple PK is enough.

***

## **Why this matters**

Alembic autogenerate is standard tooling for SQLAlchemy projects. BigQuery is increasingly used with SQLAlchemy (especially for ETL / analytics pipelines). Without correct constraint reflection, **migrations cannot be generated at all**.

This blocks:

* new project bootstrapping
* schema evolution workflows
* compatibility with SQLAlchemy 2.x and Alembic 1.7+

***

## **Proposed solutions**

### **Short term**

Ensure dialect’s Inspector methods **always** return dictionaries with the keys Alembic expects:

* For PKs:
```python
{"name": None, "constrained_columns": [...]}
```

* For UNIQUE constraints:
```python
{"name": None, "column_names": [...]}
```

Even if BigQuery does not enforce constraints, returning a complete dict prevents Alembic from crashing.

### **Long term**

Implement actual constraint reflection using BigQuery’s `INFORMATION_SCHEMA.TABLE_CONSTRAINTS` and related views.

***

## **Workaround**

Users can monkey‑patch Inspector methods in `env.py` to inject `'name'` fields, but this is brittle and clearly not the intended solution.

***

## **Request**

Please add proper constraint reflection to the dialect, or at least ensure that Inspector methods return dictionaries compatible with Alembic expectations so that autogenerate no longer fails.

Happy to provide a PR or deeper reproducer if helpful.