test: add unit tests for the unflatten utility

[eeshsaxena]

What does this PR do?

Adds a unit test suite for bentoml/_internal/utils/unflatten.py, which previously had no test coverage anywhere in the suite.

unflatten turns flat dotted/bracketed keys into nested structures, and it has quite a few branches worth pinning down. The new tests/unit/_internal/utils/test_unflatten.py covers:

• Nested dicts ({"foo.bar": "val"} → {"foo": {"bar": "val"}})
• Nested lists, lists of lists, and lists of dicts
• Empty input, plain terminals, deep nesting, and preserved empty-string values
• Accepting an iterable of (key, value) pairs, not just a dict
• Quoted keys keeping a literal dot
• Error paths: conflicting container/terminal types (both directions), missing list indices, and non-string keys

Tests only; no library code changed. Runs green locally (17 passed) and ruff check / ruff format --check are clean on the file.

Before submitting:

• Does the Pull Request follow Conventional Commits specification naming?
• Does the change require documentation update? No - test-only.