perf: avoid double dict lookup for line profiler results

[KRRT7]

Summary

Replace if key in dict: dict[key] with single .get() call in handle_duplicate_candidate().

Part of #2132 — targeted E2E pipeline performance improvements.

Problem

handle_duplicate_candidate() used the pattern:

if key in optimized_line_profiler_results:
    value = optimized_line_profiler_results[key]

This computes the hash twice and performs two dict lookups. The dict is dict[str, str] so None is never a valid value.

Solution

Single .get() with a guard:

if (value := optimized_line_profiler_results.get(key)) is not None:

One hash, one lookup, same semantics.

Changes

• codeflash/models/models.py: Single .get() + guard instead of in + subscript
• tests/test_candidate_evaluation_context.py: 3 tests covering existing key, missing key, and no corruption

Test plan

• uv run pytest tests/test_candidate_evaluation_context.py -v
• Behavior identical — same assignment when key exists, no-op when missing