Fix DELETE_FIELD handling: silent no-op for missing fields + nested dict support in set(merge=True)

[Some1Somewhere]

Fix: DELETE_FIELD handling improvements

Summary

Two fixes for DELETE_FIELD behavior to match real Firestore:

1. DELETE_FIELD on non-existent fields should be a silent no-op

• In real Firestore, calling update() with firestore.DELETE_FIELD on a field that doesn't exist is a silent no-op. In mock-firestore, it raised a KeyError.
• Wrapped delete_by_path in _apply_deletes with try/except KeyError, matching the pattern already used by _apply_arr_deletes in the same file.

2. set(merge=True) with nested dicts containing DELETE_FIELD

• In real Firestore, set({"stats": {"student123": {"field": DELETE_FIELD}}}, merge=True) deep-merges and deletes the nested field. In mock-firestore, this raised a KeyError because apply_transformations flattened nested dicts into dot-notation keys via get_document_iterator, then tried del data[key] using the dot-notation key as a literal top-level key lookup on the still-nested dict.
• Added flatten_for_merge() helper that converts nested dicts into dot-notation keys before set(merge=True) calls update(), so the existing transformation logic works correctly.

Motivation

Batch updates that use set(merge=True) with nested dicts containing DELETE_FIELD (e.g., cleaning up fields that may or may not exist) would crash in tests using mock-firestore, even though they work fine against real Firestore.

Changes

File	Change
mockfirestore/_transformations.py	Catch KeyError in _apply_deletes, skip missing fields
mockfirestore/_helpers.py	Add flatten_for_merge() to convert nested dicts to dot-notation keys
mockfirestore/document.py	Use flatten_for_merge() in set() when merge=True
tests/test_document_reference.py	3 new tests for DELETE_FIELD on non-existent fields

Test plan

• Existing test_document_update_transformerSentinel still passes (delete existing field)
• Existing test_document_set_mergeNewValue still passes (set with merge, flat data)
• New: DELETE_FIELD on non-existent top-level field — no error, doc unchanged
• New: DELETE_FIELD on non-existent nested dot-notation path — no error, doc unchanged
• New: Mixed existing + non-existing DELETE_FIELD — existing field deleted, non-existing silently ignored
• Verified end-to-end: batch.set(ref, {"stats": {"student123": {"field": DELETE_FIELD}}}, merge=True) now works correctly