[Repo Assist] Add PreferFloats static parameter to CsvProvider

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Implements the feature requested in #838 — a new PreferFloats static parameter for CsvProvider that opts in to float inference over decimal.

Why

By default, CsvProvider infers decimal for numeric columns with fractional values (e.g. 9.99). While decimal offers exact arithmetic, it is incompatible with many .NET math APIs (sqrt, sin, etc.) and causes friction when the caller needs float. This parameter lets users opt in to float globally for a provider.

What changes

File	Change
StructuralInference.fs	Add preferFloats: bool to inferPrimitiveType; guard decimal arm with when not preferFloats; add internal getInferedTypeFromStringPreferFloats
CsvInference.fs	Thread preferFloats through inferCellType, inferType, inferColumnTypes, CsvFile.InferColumnTypes
HtmlInference.fs	Pass false (HTML provider unaffected)
CsvProvider.fs	Register PreferFloats static parameter at index 19
TypeProviderInstantiation.fs	Update CsvProviderArgs test record
CsvProvider.fs (tests)	Add two tests

Usage

// Default: columns with 9.99-style values infer as decimal
type PricesDefault = CsvProvider<"prices.csv">
// prices.Price : decimal

// With PreferFloats=true: same columns infer as float
type PricesFloat = CsvProvider<"prices.csv", PreferFloats = true>
// prices.Price : float

The Schema parameter continues to override inference for individual columns regardless of PreferFloats.

Trade-offs

• decimal provides exact arithmetic; float does not. Users should set PreferFloats=true only when they know they need float compatibility (e.g., for math functions).
• This is a purely additive, opt-in change. Default behaviour is unchanged.

Test Status

All 486/487 design-time tests pass (1 skipped pre-existing). All 68 CsvProvider tests pass (66 existing + 2 new).

Closes #838

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