fix(core): applied `secrets_map` in `load` to plain string values (#33678)

Replaces #33618 

**Description:** Fixes the bug in the `load()` function where secret
placeholders in plain dicts were not replaced, even if they match a key
in `secrets_map`, and adds a test case.

Example:
```py
obj = {"api_key": "__SECRET_API_KEY__"}
secret_key = "secret_key_1234"
secrets_map = {"__SECRET_API_KEY__": secret_key}
result = load(obj, secrets_map=secrets_map)
```
Before this change, printing `api_key` in `result` would output
`"__SECRET_API_KEY__"`. Now, it will properly output
`"secret_key_1234"`.

**Issue:** Fixes #31804 

**Dependencies:** None

`make format`, `make lint`, and `make test` have all passed on my
machine.

---------

Co-authored-by: Mason Daugherty <mason@langchain.dev>
Co-authored-by: Mason Daugherty <github@mdrxy.com>