Preserve within-file duplicates and simplify matching logic

[danischm]

Summary

Improves YAML list merging behavior to preserve within-file duplicates while simplifying the
matching logic. This makes the behavior more predictable and intuitive.

Changes

1. Preserve Within-File Duplicates (Order-Independent)

Previously, deduplicate=True would remove ALL duplicate list items globally, including
duplicates within a single file.

New behavior: If ANY file contains duplicates in a list, that entire list is concatenated
(no merging) to preserve all duplicates. This ensures:

• ✅ All within-file duplicates preserved (from any file)
• ✅ Order-independent results (same output regardless of file load order)
• ✅ Predictable behavior ("duplicates disable merging")

Example:

# file1.yaml
devices:
  - name: switch1
  - name: switch1  # duplicate

# file2.yaml
devices:
  - name: switch1
    ip: 192.168.1.1

Before: 1 device (file1 duplicates lost)
After: 3 devices (all preserved, no merging)

2. Relaxed Matching Logic

Previously, dict items would NOT merge if both sides had unique primitive keys. This prevented
useful scenarios like combining complementary configuration data.

New behavior: Items merge as long as they share at least one primitive key with matching values.
Both sides can have unique keys - they'll be combined.

Example:

# file1.yaml
devices:
  - name: switch1
    vlan: 100

# file2.yaml
devices:
  - name: switch1
    port: eth0

Before: 2 separate items (both have unique keys)
After: 1 merged item {name: switch1, vlan: 100, port: eth0}

Implementation Details

• Added _has_duplicates_in_list() helper function to detect duplicates using same matching logic
• Modified merge_dict() to check both source and destination lists for duplicates before merging
• Updated docstrings with clear examples of new behavior

Breaking Changes

1. Duplicate preservation: Lists containing duplicates in ANY file will now be concatenated
   instead of merged, preserving all within-file duplicates. This may result in more items than
   before if you have duplicates and previously relied on cross-file merging.

2. Relaxed matching: Items now merge when they share primitive keys, even if both sides have
   unique keys. Complementary configuration data will be combined instead of kept separate. Items
   that previously stayed separate may now merge.

Testing

• ✅ All existing unit tests pass (with updated expectations)
• ✅ New test cases for both-sides-unique-keys scenario
• ✅ Verified duplicate preservation from any file
• ✅ Verified order-independence
• ✅ Ruff linting passes
• ✅ Mypy type checking passes