[Server] Always emit items for array tool parameter schemas

[valeriudev]

Problem

VS Code / Copilot and other strict MCP clients reject tools whose input schema declares an array parameter without an items keyword:

Failed to validate tool <name>: Error: tool parameters array type must have items. Please open a Github issue for the MCP server or extension which provides this tool

Any handler with an untyped array parameter (e.g. array $params = [], as in the report) generated {"type": "array"} with no items, so the tool was unusable in those clients.

Root cause

SchemaGenerator only set items when it could infer an element type:

• Untyped arrays (array, array<string, mixed> maps, mixed) and untyped variadics emitted type: array with no items at all.
• A nullable typed array (string[]|null) lost its element type entirely, because the |null suffix wasn't stripped before the T[] inference ran — so it too fell back to no items.

Fix

src/Capability/Discovery/SchemaGenerator.php, two parts:

1. ensureArrayItems() runs after all schema merges (and on the variadic path). When the schema is array-typed and still has no items, it defaults to the empty schema — items: {} (a \stdClass, so it serializes to {} and not []), which matches any element. Running post-merge means a #[Schema] attribute that reshapes a parameter (e.g. to object) is respected and the invariant is only enforced on what is genuinely still an array.
2. inferArrayItemsType() strips a top-level nullable union (string[]|null, null|int[]) before inference, so nullable typed arrays recover their element type. Inner unions like array<int|string> are left untouched (the strip is anchored to the start/end only); array nullability is still handled separately via allows_null.

No public API symbol changes — this corrects derived schema output to be spec-compliant, so it is a bugfix, not a [BC Break].

Verification

• New unit tests testRecoversItemsTypeForNullableTypedArrays and testUntypedVariadicStillDeclaresItems; tightened the existing array-case assertions; new fixtures nullableTypedArrays / untypedVariadic.
• make tests (unit + inspector), make phpstan, make cs, composer validate --strict, and make docs all green.
• End-to-end: the committed HttpComplexToolSchemaTest snapshot now shows the nullable string[]|null attendee-emails parameter correctly carrying items: {type: string}.

Note for merge

A parallel branch of mine also opens a 0.6.1 CHANGELOG section. If it lands first, I'll rebase and move this bullet under the existing 0.6.1 heading rather than re-adding it.

Fixes #151