[Flagd] Fix and re-enable reconnect logic in RPC mode e2e tests

[alexandraoberaigner]

The reconnect logic in the RPC mode is currently excluded from e2e tests by the ~@reconnect tag in providers/flagd/e2e/rpc_test.go. Recent test runs show failures when this exclusion is removed.
Update the reconnect implementation to pass all related e2e tests, then remove && ~@reconnect from the test tags to re-enable these tests.

Acceptance Criteria:

• Reconnect logic is fixed and pass its e2e tests
• The @reconnect tag is no longer excluded in rpc_test.go.

File to update:
providers/flagd/e2e/rpc_test.go - Remove reconnect exclusion
RPC provider implementation files - Fix reconnect logic

Additional context:
See recent test failures in TestRPCProviderE2E for details on reconnect issues.

---

Supersedes #411