WIP: iceberg: reset coordinator pending files

[andrwng]

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v25.3.x
• v25.2.x
• v25.1.x
• v24.3.x

Release Notes

• None

[Copilot]

Pull request overview

This PR adds functionality to reset pending files in the Iceberg coordinator. The change introduces a new reset_pending_state operation that clears pending entries for a topic's partitions while validating topic revision.

Key changes:

• New RPC endpoint coordinator_reset_pending_state exposed via admin API
• New state machine update type reset_pending_state for clearing pending files
• Frontend and service layer implementations for the reset operation

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
datalake.h/cc	Adds admin service handler for reset pending state (contains merge conflict)
types.h	Defines request/reply types for reset pending state operation
state_update.h/cc	Implements reset_pending_state_update with validation and apply logic
state_machine.cc	Adds state machine handling for reset_pending_state updates
service.h/cc	Implements RPC service endpoint for reset pending state
rpc.json	Registers new RPC method in service definition
frontend.h/cc	Adds frontend layer handling for reset operations
coordinator.h/cc	Implements coordinator logic with STM replication and validation
datalake.proto	Defines protobuf service and messages for admin API