fix(mdds): log text-column numeric coercion so wire-type drift stays observable

[userFRM]

The list endpoints publish their values as numeric cells while every binding presents the result as a string list: list_dates and list_expirations carry YYYYMMDD as Number cells, and list_strikes carries the strike as a Number or Price cell. extract_text_column therefore coerces numeric cells to strings, and sorted_list_values applies a numeric-aware sort on top. Rejecting numeric cells with a typed error here would break those served endpoints on every call, so the coercion is intentional rather than a swallowed type drift.

Previously the coercion was fully silent, so a text column the server unexpectedly starts publishing as numeric left no trace at all. Each coercion now emits a tracing::trace! naming the requested header and the observed wire variant. The level is trace rather than warn because numeric cells are the expected shape on the numeric list endpoints, so a higher level would be pure noise; an unexpected drift on a genuinely textual column is still recoverable from a trace-enabled run.

While in the file, the sorted_list_values numeric-sort f64::MAX fallbacks are now documented as unreachable: the all_numeric gate proves every element parses as a finite f64 and the vector is not mutated before the sort, so the unwrap_or branches only keep the comparator total without re-asserting the gate's invariant. No behavior change there.

Added a unit test confirming a Number cell in a text-requested column coerces to its decimal string rather than dropping to None.

Verification: cargo test -p thetadatadx --lib mdds (184 passed) and --features __internal extract (15 passed, including the new test); cargo fmt --all -- --check; cargo clippy -p thetadatadx --all-targets --locked --features __internal -- -D warnings clean.

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