Add validation package with http and group subpackages

[JAORMX]

Summary

• Graduate the validation package from toolhive with a structured subpackage layout
• Third Tier 1 package per THV-0032 RFC

validation/http

Security-focused HTTP validation per RFC 7230 and RFC 8707:

import "github.com/stacklok/toolhive-core/validation/http"

http.ValidateHeaderName("X-API-Key")           // RFC 7230 header names
http.ValidateHeaderValue("Bearer token")       // RFC 7230 header values
http.ValidateResourceURI("https://api.example.com")  // RFC 8707 resource URIs

Protects against:

• CRLF injection attacks
• Control character injection
• Malformed URIs

validation/group

Group name validation for consistent naming conventions:

import "github.com/stacklok/toolhive-core/validation/group"

group.ValidateName("my-team-123")  // lowercase alphanumeric, dashes, underscores, spaces

Test plan

• task lint passes
• task license-check passes
• task test passes with race detection

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