Validate options at startup instead of throwing NullReferenceException

[matthewdevenny]

Problem

A missing/empty BucketName throws a NullReferenceException from the NatsCache constructor (src/NatsDistributedCache/NatsCache.cs). This surfaces as a confusing error at first use rather than a clear configuration failure.

Proposal

Validate options at startup using the options pattern:

services.AddOptions<NatsCacheOptions>()
    .Validate(o => !string.IsNullOrWhiteSpace(o.BucketName), "BucketName must be set")
    .ValidateOnStart();

(or an IValidateOptions<NatsCacheOptions> implementation). Remove the NullReferenceException throw.

Acceptance criteria

• Empty/whitespace BucketName fails fast at startup with a descriptive OptionsValidationException.
• No NullReferenceException path remains.

Good first issue.