feat(ws): reject invalid requests

[Yael-F]

Fixes #129 by implementing validation for names and namespaces.

Summary of Changes

Validation Enhancements:

Ensures names/namespaces extracted from URL parameters or request payloads are:

• ASCII-compliant.
• Within the allowed length (≤ 255 characters).

New Validation Module:
Introduced functions for validation:
NonASCIIValidator
LengthValidator
ValidateKubernetesResourceName

Handler Updates:
Applied validation to the following routes:
GET /workspacekind/:name
GET /workspaces/:namespace/:name
GET /workspaces/:namespace
POST /workspaces/:namespace

Testing:
Updated test files (workspaces_handler_test.go, workspacekinds_handler_test.go) to ensure unsupported fields trigger validation errors.

[google-oss-prow]

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[ederign]

@Yael-F can you please rebase this?