MigrationWriter silently serializes Python 3.15 frozendict values as tuples of keys

[amanjain]

Ticket title:

`
MigrationWriter silently serializes Python 3.15 frozendict values as tuples of keys
`

Ticket description:

On Python 3.15.0, Django's migration serializer treats built-in frozendict instances as generic iterables, preserving only their keys and silently discarding their values.

Reproduced against Django main at d430084791.

Minimal reproducer:

`python
from django.db.migrations.writer import MigrationWriter

MigrationWriter.serialize(frozendict(answer=42))
`

Actual result:

`python
("('answer',)", set())
`

Expected behavior: preserve the mapping's keys and values, rather than serialize it as a tuple of keys.

This affects field defaults. For example:

`python
models.JSONField(default=frozendict(answer=42))
`

is written into a migration as:

`python
models.JSONField(default=('answer',))
`

I reproduced the database consequence on SQLite: after executing a generated AddField migration against a table containing an existing row, the new JSON field contains answer instead of {"answer": 42}.

The serializer registry handles dict explicitly, but frozendict is not a dict subclass and falls through to IterableSerializer.

A proposed patch adds a dedicated frozendict serializer, regression tests in the existing migration writer and operation suites, and documentation noting that generated migrations containing frozendict require Python 3.15+.

Validation:

• Python 3.15: all 75 writer/regression tests passed.
• Python 3.15: 181 migration operation tests ran successfully, with 2 skipped.
• Python 3.14: 75 writer/regression tests ran successfully, with 3 skipped.
• Formatting and whitespace checks passed.
• The full Django test suite has not been run.

AI disclosure: OpenAI Codex (GPT-6) assisted with investigation, implementation, regression tests, documentation, and commit preparation.