fix: enable SSE reconnect in multi-user client

[vazarkevych]

Problem

In multi-user mode, GrowthBookClient always called the no-arg
repository.initialize() (in initializeFeaturesRepository()), which delegates to
initialize(false). That hardwires retryOnFailure = false, so
scheduleSseReconnect(...) early-returns and the SSE reconnect machinery is unreachable.

As a result, a GrowthBookClient using FeatureRefreshStrategy.SERVER_SENT_EVENTS
permanently stops receiving feature updates after the first abnormal stream failure
(connection reset, proxy/pod restart, mid-stream IOException) — silently:

• no reconnect attempt;
• no polling fallback (schedulePolling() early-returns for SSE);
• no observability — FeatureRefreshCallback.onError(...) never fires.

Both onFailure and onClose route through scheduleSseReconnect, so multi-user + SSE
also stops reconnecting on a graceful close. Options.retryPolicy (exponential
backoff) was configurable but never actually engaged.

Fix

Expose the flag on Options and thread it through the single choke point:

• Options.sseReconnectOnFailure — new option, defaults to true, with
  isSseReconnectOnFailure() returning true unless explicitly disabled. Added as the
  last constructor parameter to preserve the existing positional constructor signature.
• GrowthBookClient.initializeFeaturesRepository() now calls
  repository.initialize(options.isSseReconnectOnFailure()) instead of the no-arg form.

Reconnect is on by default (bounded by retryPolicy), and callers can opt out with
Options.builder().sseReconnectOnFailure(false). The single-user
GBFeaturesRepository.initialize() default (false) is unchanged to avoid a broader
public-API behavior change.

Tests

backoff) was configurable but never actually engaged.

Fix

Expose the flag on Options and thread it through the single choke point:

• Options.sseReconnectOnFailure — new option, defaults to true, with
  isSseReconnectOnFailure() returning true unless explicitly disabled. Added as the
  last constructor parameter to preserve the existing positional constructor signature.
• GrowthBookClient.initializeFeaturesRepository() now calls
  repository.initialize(options.isSseReconnectOnFailure()) instead of the no-arg form.

Reconnect is on by default (bounded by retryPolicy), and callers can opt out with
Options.builder().sseReconnectOnFailure(false). The single-user
GBFeaturesRepository.initialize() default (false) is unchanged to avoid a broader
public-API behavior change.