battery: Add get-bix and set-btp support

[kurtjd]

Need to support get-bix and set-btp since these are required by our EC test apps. Tested with the test-cli app within th WinVOS image running on QEMU.

Resolves #93

[Copilot]

🟢 Approval recommended

Implements the required commands with explicit serialization/dispatch tests and a minimal relay-side buffer adjustment to support the larger response size.

Pull request overview

This PR adds the missing battery relay functionality required by the EC test apps by extending the Battery FFA service to support GetBix and SetBtp in addition to the existing GetBst path.

Changes:

• Introduces a typed BatteryCommand enum and adds get_bix / set_btp relay methods with fixed-size decoding/encoding.
• Extends the FFA direct-request handler to dispatch GetBix, GetBst, and SetBtp.
• Adds wire-format compatibility tests for GetBix and SetBtp, and increases the EC relay RX packet buffer to accommodate larger replies.

File summaries

File	Description
ec-service-lib/src/services/ec_relay.rs	Enlarges the per-call RX packet buffer to handle larger framed responses (needed for BIX-sized payloads).
ec-service-lib/src/services/battery.rs	Adds GetBix/SetBtp command support end-to-end (relay + FFA dispatch) with gate tests to lock the wire format.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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