refactor!: consolidate optional send*/handle* parameters into ContextList

[william-suppo]

Summary

• Closes Consolidate optional send*/handle* parameters into a typed ContextList #12. send*/handle* methods on IdentityProviderWrapper and ServiceProviderWrapper had accumulated one optional parameter per protocol detail (relayState, sessionIndex, nameIdPolicyFormat, nameId), plus bool $validate / ?Entity $issuer duplicated across 8 call sites.
• Introduces Litesaml\Models\Messages\Context\{Context, ContextList} — a marker interface and an immutable, variadic, class-keyed collection (all(), first(), required()) — and collapses every optional parameter into a single trailing ContextList $context = new ContextList().
• New context value objects: RelayState, SessionIndex, NameIdPolicyFormat, Validate(Entity $issuer).
• Validate merges the old bool $validate + ?Entity $issuer pair. Since $issuer is a required constructor argument, "validate without an issuer" is no longer representable, so that runtime exception path (and its tests) is removed.
• NameId and Attribute move from Litesaml\Models\Messages to Litesaml\Models\Messages\Context and now implement Context.

Breaking changes

• NameId/Attribute namespace change — update use statements.
• All send*/handle* signatures collapse to (..., ContextList $context = new ContextList()); existing positional/named-argument call sites break.
• sendLogoutRequest()'s NameId is no longer statically required — a ContextList missing one now throws SamlException at call time (ContextList::required()) instead of failing at the type level.
• bool $validate / ?Entity $issuer parameters removed in favor of new Validate($issuer).

Test plan

• composer test (68 tests, 175 assertions)
• composer phpstan — no errors