<!-- Thank you for your contribution! -->
<!-- Unless your change is trivial, please create an issue to discuss the change before creating a PR -->

## Change Summary

Adds these types:

```python
JsonValue: TypeAlias = int | float | str | bool | None | List['JsonValue'] | 'JsonDict'
JsonDict: TypeAlias = Dict[str, JsonValue]
```

and replaces `dict[str, object]` with `JsonDict` on annotations of `json_schema_extra`.

## Related issue number

Closes https://github.com/pydantic/pydantic/issues/6348

Replaces https://github.com/pydantic/pydantic/pull/7475 which seems to have died and implements the suggestions.

<!-- WARNING: please use "fix #123" style references so the issue is closed when this PR is merged. -->

## Checklist

* [x] The pull request title is a good summary of the changes - it will be used in the changelog
* [x] Unit tests for the changes exist
* [x] Tests pass on CI
* [x] Documentation reflects the changes where applicable
* [x] My PR is ready to review, **please add a comment including the phrase "please review" to assign reviewers**


Selected Reviewer: @lig
Add more specific type for non-callable `json_schema_extra`. <!-- Thank you for your contribution! -->
<!-- Unless your change is trivial, please create an issue to discuss the change before creating a PR -->

## Change Summary

Adds these types:

```python
JsonValue: TypeAlias = int | float | str | bool | None | List['JsonValue'] | 'JsonDict'
JsonDict: TypeAlias = Dict[str, JsonValue]
```

and replaces `dict[str, object]` with `JsonDict` on annotations of `json_schema_extra`.

## Related issue number

Closes https://github.com/pydantic/pydantic/issues/6348

Replaces https://github.com/pydantic/pydantic/pull/7475 which seems to have died and implements the suggestions.

<!-- WARNING: please use "fix #123" style references so the issue is closed when this PR is merged. -->

## Checklist

* [x] The pull request title is a good summary of the changes - it will be used in the changelog
* [x] Unit tests for the changes exist
* [x] Tests pass on CI
* [x] Documentation reflects the changes where applicable
* [x] My PR is ready to review, **please add a comment including the phrase "please review" to assign reviewers**


Selected Reviewer: @lig
"""
Test mypy failure with invalid types.
"""
from datetime import datetime
from typing import List, Optional

from pydantic import BaseModel


class Model(BaseModel):
    age: int
    first_name = 'John'
    last_name: Optional[str] = None
    signup_ts: Optional[datetime] = None
    list_of_ints: List[int]


m = Model(age=42, list_of_ints=[1, '2', b'3'])

print(m.foobar)

"""
Test mypy failure with invalid types.
"""
from datetime import datetime
from typing import List, Optional

from pydantic import BaseModel


class Model(BaseModel):
    age: int
    first_name = 'John'
    last_name: Optional[str] = None
    signup_ts: Optional[datetime] = None
    list_of_ints: List[int]


m = Model(age=42, list_of_ints=[1, '2', b'3'])

print(m.foobar)
