Regex objects not supported

### Description
Docstring of `esm_datastore.search` has an example using `re.compile(...)`. However, this support seems to have broken in the last updates.

```python3
import intake

cat = intake.open_esm_datastore('intake-esm/tutorial-catalogs/AWS-CMIP6.json')

# Get institutions that do not start with M
cat.search(institution_id=re.compile('^(?!M.*)')
```
Fails with :
```python3
TypeError Traceback (most recent call last)
Cell In[37], line 1
----> 1 cat2.search(institution_id=re.compile('^(?!M.*)')).df

File ~/miniforge3/envs/intesm-dev/lib/python3.14/site-packages/pydantic/_internal/_validate_call.py:40, in update_wrapper_attributes..wrapper_function(*args, **kwargs)
38 @functools.wraps(wrapped)
39 def wrapper_function(*args, **kwargs):
---> 40 return wrapper(*args, **kwargs)

File ~/miniforge3/envs/intesm-dev/lib/python3.14/site-packages/pydantic/_internal/_validate_call.py:137, in ValidateCallWrapper.__call__(self, *args, **kwargs)
134 if not self.__pydantic_complete__:
135 self._create_validators()
--> 137 res = self.__pydantic_validator__.validate_python(pydantic_core.ArgsKwargs(args, kwargs))
138 if self.__return_pydantic_validator__:
139 return self.__return_pydantic_validator__(res)

File ~/Projets/intake-esm/intake_esm/core.py:462, in esm_datastore.search(self, require_all_on, **query)
406 """Search for entries in the catalog.
407
408 Parameters
(...) 458 4 landCoverFrac
459 """
461 # step 1: Search in the base/main catalog
--> 462 esmcat_results = self.esmcat.search(require_all_on=require_all_on, query=query)
464 # step 2: Search for entries required to derive variables in the derived catalogs
465 # This requires a bit of a hack i.e. the user has to specify the variable in the query
466 derivedcat_results = []

File ~/Projets/intake-esm/intake_esm/cat.py:443, in ESMCatalogModel.search(self, query, require_all_on)
415 """
416 Search for entries in the catalog.
417
(...) 432
433 """
435 _query = (
436 query
437 if isinstance(query, QueryModel)
(...) 440 )
441 )
--> 443 results = search(
444 df=self.df, query=_query.query, columns_with_iterables=self.columns_with_iterables
445 )
446 if _query.require_all_on is not None and not results.empty:
447 results = search_apply_require_all_on(
448 df=results,
449 query=_query.query,
450 require_all_on=_query.require_all_on,
451 columns_with_iterables=self.columns_with_iterables,
452 )

File ~/Projets/intake-esm/intake_esm/_search.py:46, in search(df, query, columns_with_iterables)
42 column_is_stringtype = isinstance(
43 df[column].dtype, object | pd.core.arrays.string_.StringDtype
44 )
45 column_has_iterables = column in columns_with_iterables
---> 46 for value in values:
47 if column_has_iterables:
48 mask = df[column].str.contains(value, regex=False)

TypeError: 're.Pattern' object is not iterable
```

### Case with PyArrow
However, I don't think this is fixable. When opening a catalog from a csv file, the resulting dataframe has string columns with a `large_string[pyarrow]` dtype. Pandas will then delegate the pattern matching to pyarrow, which doesn't support re objects.

```python3
cat = intake.open_esm_datastore('intake-esm/tests/sample-catalogs/cesm1-lens-netcdf.json')
cat.df.experiment.str.contains(re.compile('^C.*'))
```
fails with `TypeError: expected bytes, re.Pattern found`

Moreover, pyarrow uses a different regex than python. It uses Google RE2. A major difference (to me atleast) is the absence of negative matches in Google RE2. For example, `^(?!CCCma.*)` (match strings not starting with "CCCma") is not a valid pattern.

### How to fix
I think we could simply remove the example using `re.compile` from the documentation and note somewhere that because of PyArrow usage, intake-esm's search function only officially supports the intersection between python's and Google RE2's regex syntaxes.

### Version information: output of `intake_esm.show_versions()`

Paste the output of `intake_esm.show_versions()` here:

```python
INSTALLED VERSIONS
------------------

cftime: 1.6.5
dask: 2026.3.0
fastprogress: 1.1.3
fsspec: 2026.2.0
gcsfs: 2026.2.0
intake: 2.0.9
intake_esm: 2025.12.12.post7+g414c4cfc1
netCDF4: 1.7.4
pandas: 3.0.2
requests: 2.33.1
s3fs: 2026.2.0
xarray: 2026.4.0
zarr: 3.1.6
```

Start with the esm_datastore.search docstring in intake_esm/core.py, then read the search path in intake_esm/cat.py and intake_esm/_search.py. Remove the re.compile example and document the supported regex limitations described in the issue. Confirm the documentation no longer promises unsupported Python regex behavior.

Written by the indexing model from the issue text.