feat(core): Introduce ErrNoValue for nuanced validation

[FranciscoKurpiel]

This commit introduces a new mechanism to allow xtypes to signal that a provided value (such as an empty string) should be treated as if no value was provided at all. This solves a design limitation where the validation logic could not distinguish between a valid empty value and a "not provided" marker for certain types.

Problem:

Previously, an xtype like xtypes.RSAPrivateKey would fail validation when an optional parameter was backed by an environment variable set to an empty string.

• The validation function (ValueValid) is context-agnostic and does not know if a parameter is optional.
• If ValueValid returned an error for an empty string, it would incorrectly fail optional parameters.
• If ValueValid did not return an error, it would incorrectly allow a required parameter to be considered valid while producing a nil value, without MustParse returning an error.

Solution:

This commit implements a sentinel error-based solution:

1. types.ErrNoValue: A new exported error, ErrNoValue, has been added to the types package.

2. xtype Adoption: xtypes that need this behavior (like RSAPrivateKey) can now return types.ErrNoValue from their ValueValid function when they encounter a value (like an empty string) that should be treated as "not provided".

3. Core Logic Update: The central validation function validValue in parsed.go has been updated to recognize ErrNoValue. When it receives this error, it now correctly applies the optional/required logic as if the parameter had not been supplied at all.

Consequences:

• For Users: This change fixes the bug where optional RSAPrivateKey parameters would fail with empty environment variables. The behavior is now consistent and correct. The default value of an optional parameter is now correctly used when an empty string is provided.
• For xtype Developers: Developers of new xtypes now have a clear, idiomatic pattern to handle cases where certain string representations should be considered "empty" or "not provided". They can return types.ErrNoValue from ValueValid to trigger this behavior.
• Testing: A comprehensive test case (TestRSAPrivateKey) has been added to parser_test.go to verify this new behavior and prevent future regressions.

This change makes the validation system more robust and flexible while maintaining safety and correctness.

[FranciscoKurpiel]

/gemini review