Nullness FS3261: pinpoint receiver range, name the member & binding in dot-access warnings

[T-Gro]

Fixes #19658

Architecture

New ContextInfo.MemberAccessOnNullable threads receiver range + member name + optional binding name through constraint solving. When FS3261 fires during dot-access, a dedicated ConstraintSolverNullnessWarningOnDotAccess emits a targeted diagnostic instead of the generic "types do not have compatible nullability".

Before → After

1. Simple method call on nullable receiver

let pad (x: string | null) = x.PadLeft(10)

• Before: FS3261 on x.PadLeft(10) — Nullness warning: The types 'string' and 'string | null' do not have compatible nullability.
• After: FS3261 on x — Nullness warning: Possible dereference of a null value when accessing member 'PadLeft' on the nullable value 'x' of type 'string | null'.

2. Chained call — warning targets the actual nullable hop

let f (x: string | null) = x.Trim().Length

• Before: Two FS3261 on x.Trim().Length and x.Trim() — Nullness warning: The types 'string' and 'string | null' do not have compatible nullability.
• After: Single FS3261 on x — Nullness warning: Possible dereference of a null value when accessing member 'Trim' on the nullable value 'x' of type 'string | null'.

3. Multi-line receiver gets "See also" pointing at call site

let process (data: string | null) =
    data
     .ToUpper()

• Before: FS3261 on .ToUpper() — Nullness warning: The types 'string' and 'string | null' do not have compatible nullability.
• After: FS3261 on data — Nullness warning: Possible dereference of a null value when accessing member 'ToUpper' on the nullable value 'data' of type 'string | null'. See also <line with .ToUpper()>

4. Property chain — warning pinpoints where nullability starts

// Given: type Foo with member Prop : Bar | null
let g (foo: Foo) = foo.Prop.ToString()

• Before: FS3261 on foo.Prop.ToString() — Nullness warning: The types 'Bar' and 'Bar | null' do not have compatible nullability.
• After: FS3261 on foo.Prop — Nullness warning: Possible dereference of a null value when accessing member 'ToString' on a nullable expression of type 'Bar | null'.

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Entity.Typars cleanup (included because it fixes a parallel test race exposed by the new tests)

Entity.Typars changed from method Typars(m: range) to property. The range parameter was unused for F# types (always NotLazy — typars carry their own ranges from unpickle) and caused a parallel test race for IL-imported types (shared BCL entities via FrameworkImportsCache + LazyWithContext caching first caller's range). Now uses entity.Range deterministically. Removed redundant TyparsNoRange.

[github-actions]

❗ Release notes required

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Change path	Release notes path	Description
src/Compiler	docs/release-notes/.FSharp.Compiler.Service/11.0.100.md	No current pull request URL (#19814) found, please consider adding it