Expand BPX GitHub file tests to cover all fields with warnings for mapping gaps

[Copilot]

test_bpx_github_files.py only spot-checked ~7 fields per conversion, leaving the majority of each parameter set untested and giving no signal about fields silently dropped due to missing ontology mappings.

Changes

Three new test helpers

• _get_nested_value(data, path) — traverse a nested dict by tuple path, returning None on any missing key
• _check_all_mapped_fields(bpx_raw, converted, mappings, output_type) — iterates every field in bpx_raw["Parameterisation"]; hard-asserts mapped fields are present with correct values (numeric: 1 ppm tolerance; string/function: non-empty expression); emits UserWarning for each unmapped field
• _check_round_trip(converted, reference, mapped_output_paths) — compares converted output against a reference along every mapped path; emits UserWarning on key or value mismatch

Updated test classes

Spot-check methods in TestNMCBPXToBattMo, TestLFPBPXToBattMo, TestChen2020BPXToBattMo, and TestChen2020BattMoToBPX replaced with comprehensive tests:

def test_all_fields(self):
    """Check every BPX field; warn about unmapped ones, assert mapped ones."""
    _check_all_mapped_fields(
        self.bpx_raw, self.result, self.mappings, "battmo.m"
    )

Warning behaviour

Running with -W default::UserWarning now surfaces 33 actionable warnings:

• 32 BPX field ... has no mapping to 'battmo.m' — the 16 known-unmapped fields per BPX file (thermal params, transport efficiency, entropic coefficients; see WORKAROUNDS.md)
• 1 OCP string whitespace difference in the BattMo→BPX round-trip (minor normalisation difference between the two source files)

Tests hard-fail only on genuine mapping regressions; gaps in the ontology are visible but non-blocking.

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