[release/11.0] Make the Nullable<T>.Value exception message actionable - #132379

Fixes #126560

## Problem

`InvalidOperation_NoValue` — the message thrown by `Nullable<T>.Value`
when there's no value — reads:

> Nullable object must have a value.

As #126560 points out, this states an obligation on the *object's state*
rather than naming the *invalid action*. It reads as "you must assign a
value", when the actual fix is almost always the opposite: don't read
`.Value` without checking `.HasValue` first. It also never mentions
which member was accessed.

## Change

```diff
-Nullable object must have a value.
+Cannot read the Value property of a Nullable object that has no value. Check HasValue before reading Value.
```

This follows the wording style already used in the repo for comparable
"you called this on something that has no value" failures:

- `SqlMisc_NullValueMessage` (`System.Data.Common`) — *"Data is Null.
This method or property cannot be called on Null values."*
- `ReflectionModel_ExportNotReadable`
(`System.ComponentModel.Composition`) — *"Cannot get the value of
property '{0}', because the member is not readable. The property must
have an accessible getter."*
- `NoResultOnFailed` (`System.Text.RegularExpressions`) — *"Result
cannot be called on a failed Match."*

The issue also floated "Cannot read the Value property of a null
Nullable object." I avoided "null Nullable object" because `Nullable<T>`
is a struct and is never `null` — "that has no value" keeps the same
cadence without the imprecision. The second sentence names the check to
perform, per *"DO ensure that exception messages are clear and
actionable"*
(`docs/coding-guidelines/framework-design-guidelines-digest.md`).

The resource **name** is unchanged.

## Scope

`InvalidOperation_NoValue` is not shared with any unrelated call site —
all definitions and references are `Nullable<T>.Value` semantics:

| | Path |
|---|---|
| def |
`src/libraries/System.Private.CoreLib/src/Resources/Strings.resx` |
| def |
`src/coreclr/nativeaot/System.Private.Reflection.Execution/src/Resources/Strings.resx`
|
| use |
`ThrowHelper.ThrowInvalidOperationException_InvalidOperation_NoValue` —
sole caller is the `Nullable<T>.Value` getter |
| use | `CustomMethodMapper.Nullable.cs` — NativeAOT reflection-invoke
shim for `Nullable<T>.Value` |

Both `.resx` definitions are updated. Mono shares the libraries CoreLib
`.resx`; there is no third copy. No test asserts the text, and the
NativeAOT `FrameworkStrings` string-pinning smoke test pins other
resources.

Per `docs/coding-guidelines/breaking-change-rules.md`, changing the text
of an error message is not a breaking change ("users should not rely on
these text messages, and they change anyways based on culture").

## Validation

- Built `System.Private.CoreLib` (`build.cmd clr.corelib -c Release`) —
clean.
- Confirmed against the produced `System.Private.CoreLib.dll` that the
new string is embedded and the old one is gone, so
`SR.InvalidOperation_NoValue` still resolves.
- The NativeAOT `System.Private.Reflection.Execution` `.resx` was not
compiled locally — that project currently fails on a pre-existing
missing generated `AsmOffsets.cs` requiring a full NativeAOT native
build. The edit there is identical in shape and doesn't change the
resource name. CI covers it.

> [!NOTE]
> This pull request description was generated by GitHub Copilot.

---------

Co-authored-by: Copilot App <[email protected]>
Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: jkotas <[email protected]>

Co-authored-by: Copilot App <[email protected]>