Support analyzer 14 (for IO-Design-Team/hive_ce#308)

## Context

Requested from https://github.com/IO-Design-Team/hive_ce/issues/308 (`hive_ce_generator` analyzer 14 upgrade).

This continues / supersedes https://github.com/google/source_helper.dart/issues/72 and https://github.com/google/source_helper.dart/issues/73 (opened earlier without the cross-repo link; please close those as duplicates of this issue).

## Request

Please bump the `analyzer` constraint to allow analyzer 14:

```yaml
analyzer: '>=9.0.0 <15.0.0'
```

Latest `source_helper` 1.3.12 still has `analyzer: '>=9.0.0 <14.0.0'`, which blocks dependents that need analyzer 14.

A constraint-only bump was validated locally with analyzer 14.1.0 (`dart analyze` clean, all tests passed).

Start by locating the package dependency manifest and its current analyzer constraint. Confirm the requested upper bound permits analyzer 14, then run dart analyze and the full test suite; done means both remain clean with analyzer 14.1.0.

Written by the indexing model from the issue text.