test(mapping): add dedicated tests for compose_mappings

[protosphinx]

Why

compose_mappings is the public API for stacking event-to-symbol strategies (regex, embedding, LLM, manual). It is mentioned explicitly in the roadmap and README as the composition primitive, but its behavior is only tested indirectly in two tests at the tail of test_mapping_manual.py. This PR adds a dedicated test file covering the paths those tests miss.

What

• New file tests/test_mapping_compose.py with 9 focused tests:
  • zero-strategy composition returns all-empty results
  • single-strategy passthrough matches a direct call
  • all strategies miss for an event, returns empty list
  • three-strategy chain: second fills the gap, third result is not used
  • multiple events resolved by different strategies, with a no-match case
  • duplicate events deduplicated in the composed result
  • empty events input returns empty dict
  • empty symbol list: all strategies miss
  • result key order matches input event order

Tests

• tests/test_mapping_compose.py: 9 new tests, all green
• Full suite: 97 passed (was 88), 0 failures

Self-merge gate

• all CI checks pass
• LOC delta < 250 (added + removed)
• no public-API surface change
• no runtime-dependency additions
• no workflow file changes
• tests added or extended

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