Reject unknown keys in product selector configuration #1035

Summary

Reject unknown keys when reading a phlex::product_selector from configuration. Currently, a misspelled key is ignored and can silently widen the selector.

Requested by @beojan in PR #1024.

Current behavior

The product_selector JSON conversion in phlex/configuration.cpp reads creator, layer, suffix, and stage, but does not reject other keys.

For example:

{"creator": "finder", "layer": "event", "suffx": "seeds"}

Reading this object with config.get<phlex::product_selector>(...) ignores suffx. The suffix field remains unset instead of selecting "seeds". This can select unintended products or cause ambiguity.

Required changes

• Validate selector object keys during configuration conversion in phlex/configuration.cpp.
• Reject keys other than creator, layer, suffix, and stage.
• Report the offending key in a clear configuration error.
• Keep supported fields optional. Missing supported keys must leave their fields unset.
• Add regression coverage for direct selector conversion and selectors read within arrays.
• Update docs/source/guide/product_selectors.rst to describe the corrected behavior.

Acceptance criteria

• A selector containing suffx fails configuration conversion with a diagnostic that names suffx.
• Other unknown keys are rejected.
• Selectors containing only supported keys continue to work.
• Empty selector objects and selectors that omit supported fields remain valid.
• An unknown key in an array element is not silently ignored.

This issue concerns unknown object keys, not the separate array-length behavior documented nearby.

References

• PR: docs: document product selectors #1024
• Request: docs: document product selectors #1024 (comment)