Fix desktop app integration bugs

[lightcap]

Summary

Fixes two bugs discovered during a smoke test against the 1Password desktop app that prevented the desktop integration from working:

1. ClientConfig serialization -- The 1Password shared library requires serviceAccountToken to always be present in the config payload (even as an empty string ""), and account_name to be included when set. Previously, serviceAccountToken was omitted when empty via skip_serializing_if, and account_name was unconditionally skipped via skip_serializing. Both fields are now serialized correctly.

2. Response payload deserialization -- The shared library returns payload as a raw JSON byte array ([65,110,...]), not a base64 string. The base64_payload deserializer has been replaced with a flexible_payload deserializer that handles both formats (raw byte array and base64 string), ensuring compatibility across library versions.

Test plan

• cargo fmt --check passes
• cargo clippy -- -D warnings passes (default features)
• cargo clippy --features desktop -- -D warnings passes
• cargo test passes (23 tests, default features)
• cargo test --features desktop passes (26 tests, including 3 new/renamed desktop tests)
• Verify desktop_config_includes_account_name test confirms serviceAccountToken is "" and account_name is "myaccount"
• Verify service_account_config_omits_account_name test confirms serviceAccountToken is "ops_test" and account_name is absent
• Verify response_payload_deserializes_from_byte_array test confirms raw byte array [104,101,108,108,111] deserializes to b"hello"
• Verify response_payload_deserializes_from_base64 test still passes (backward compat)

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Note

Medium Risk
Changes request/response wire formats for the desktop shared-library integration (config serialization and response payload decoding), which can affect interoperability and error handling across library versions.

Overview
Fixes desktop shared-library interoperability by adjusting ClientConfig JSON serialization: serviceAccountToken is now always emitted (even when empty) and account_name is included when set.

Updates shared-library Response decoding to accept payload as either base64 text or a raw JSON byte array via a new flexible_payload deserializer, and updates/adds tests to cover both formats.

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