Parser: recover on missing 'when' conditions

[auduchinok]

Fixes parsing of missing when clause expressions, like the following:

match () with
| _ when -> ()

match () with
| a when
| b -> ()

[T-Gro]

🤖 AI review (@expert-reviewer): no significant issues found. Please verify independently.

Reviewed the parser recovery for missing/invalid when guard expressions and the supporting refactor:

• The four new parenPattern WHEN ... productions are correctly gated by %prec prec_recover, so they only engage when the normal WHEN declExpr reduction fails; a valid guard still parses through patternGuard. The complementary parenPattern patternGuard + recover path (guard present, result missing) remains handled by the existing patternAndGuard recover rules.
• patternResult type change (Range → range option) and ArrowRange = mArrow threading are consistent; recovered clauses correctly emit ArrowRange = None and an ArbitraryAfterError result node.
• mkMatchClauses/mkMatchClausesRecoverMissingResult preserve the original range semantics (mLastOuter |> Option.defaultValue mLast, mNextBar continuation threading), matching the updated .bsl baselines.
• Setting guard = None in the missing-expression cases is appropriate since no valid guard expression exists, and the single parsExpectingExpression diagnostic is reported at the when keyword.

Baselines (Match - When 01–04, Try - With 09) look correct.

[github-actions]

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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 (#20071) found, please consider adding it

[github-actions]

🔍 Tooling Safety Check — Affects-Compiler-Output
Affects-Compiler-Output: modifies pars.fsy and ParseHelpers (parser behavior)

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