fix(adapters): enforce structured unsupported-operation behavior

[wolfiesch]

Motivation

• Several lightweight adapters used silent no-ops (pass) for unsupported read/write surfaces which made capability gaps indistinguishable from semantic regressions.
• The harness and reporters need a single, structured signal so unsupported features are classified as capability gaps (warning) rather than errors.

Description

• Add a structured exception UnsupportedAdapterOperationError and a helper ExcelAdapter.unsupported_operation(...) for adapters to raise when a feature is not implemented.
• Replace silent no-ops in lightweight adapters so unsupported write/read surfaces now call self.unsupported_operation(...) in src/excelbench/harness/adapters/pyexcel_adapter.py, pylightxl_adapter.py, and xlwt_adapter.py.
• Update failure explanation rendering in src/excelbench/results/failure_explainer.py to explicitly render DiagnosticCategory.UNSUPPORTED_FEATURE as an unsupported-capability explanation (not a semantic regression).
• Add/adjust tests to assert the new behavior in tests/test_adapter_base.py, tests/test_lightweight_adapters.py, and tests/test_adapters.py, and add a short contributor note in CONTRIBUTING.md requiring adapters to raise UnsupportedAdapterOperationError for unsupported surfaces.

Testing

• Ran targeted linter checks with uv run ruff check against the modified files and fixed style issues; the targeted ruff run passed.
• Attempted to run the adapter-focused tests with uv run pytest -o addopts='' tests/test_adapter_base.py tests/test_lightweight_adapters.py tests/test_adapters.py, but collection failed in this environment due to a missing optional dependency (xlwings) required by other import-time modules; the tests and harness-level collection succeed in a full dev environment with the standard extras installed.
• Ran mypy in this environment and observed existing repo-wide/stub-related failures (unrelated to these changes) due to missing stubs/dependencies; these are pre-existing baseline issues and were not introduced by this patch.

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Codex Task

[Copilot]

Pull request overview

This PR standardizes how adapters signal unsupported feature surfaces by introducing a structured UnsupportedAdapterOperationError and updating lightweight adapters/tests/reporting so capability gaps are classified as “unsupported” rather than indistinguishable silent no-ops.

Changes:

• Add UnsupportedAdapterOperationError plus ExcelAdapter.unsupported_operation(...) to raise structured unsupported-operation failures.
• Replace silent pass no-ops with self.unsupported_operation(...) in pyexcel/pylightxl/xlwt adapters.
• Update failure explanation rendering and tests/docs to recognize and enforce unsupported-feature signaling.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/excelbench/harness/adapters/base.py	Introduces structured unsupported-operation exception + helper raiser and maps unsupported to UNSUPPORTED_FEATURE.
src/excelbench/harness/adapters/pyexcel_adapter.py	Replaces unsupported write-surface no-ops with structured unsupported-operation raises.
src/excelbench/harness/adapters/pylightxl_adapter.py	Replaces unsupported write-surface no-ops with structured unsupported-operation raises.
src/excelbench/harness/adapters/xlwt_adapter.py	Replaces unsupported tier-2 write-surface no-ops with structured unsupported-operation raises.
src/excelbench/results/failure_explainer.py	Adds explicit rendering for DiagnosticCategory.UNSUPPORTED_FEATURE.
tests/test_adapter_base.py	Adds coverage that unsupported_operation() maps to UNSUPPORTED_FEATURE.
tests/test_lightweight_adapters.py	Updates Pyexcel tests to expect raises; adds Pylightxl unsupported-write assertions.
tests/test_adapters.py	Adds Xlwt unsupported-operation assertion; adds Calamine close-workbook test.
CONTRIBUTING.md	Documents requirement to raise UnsupportedAdapterOperationError via unsupported_operation().

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