Complete EC service stub implementations + unit tests

Overview\n\nComplete the stub service implementations in ec-service-lib with stateful logic, error handling, and comprehensive unit tests. Each service currently has a skeleton that returns hardcoded or NotSupported responses — these need to be filled in with real state-tracking behavior.\n\n## Services\n\n- [ ] Thermal — state-tracking for all 6 opcodes (get_temperature, set/get_threshold, set/get_variable, get_policy)\n- [ ] Battery — all 15 opcodes (BTP, BMC, BST, BIX, BPC, BPS, BCT, BCV, BAR, BAT_DIS, BAT_STA, BAT_PSR, BAT_SOC, BAT_CYC, BAT_FCC)\n- [ ] FwMgmt — error handling + process_indirect\n- [ ] Notify — add/remove/assign/unassign state-tracking\n- [ ] TPM — extended handlers (get_feature_info, register, unregister, finish, deinit)\n\n## Related PRs\n\n- #56 — Thermal\n- #57 — Battery\n- #58 — FwMgmt, Notify, TPM\n\n## Scope\n\nUnit tests for each service, plus MessageHandler dispatch tests. Fixes for any bugs found (e.g. panic on invalid MessageID, shift overflow in notify).