Add live HostDB cache clearing

[bneradt]

Operators cannot discard stale HostDB entries without restarting Traffic
Server. A restart interrupts service and delays recovery from incorrect
DNS data.

This adds a traffic_ctl command and restricted JSON-RPC method to clear
HostDB safely under partition locks while preserving in-flight records.
This also uses scoped status snapshots and verifies clearing and
repopulation without a restart.

Fixes: #9022

[Copilot]

Pull request overview

This PR adds an operationally safe way to clear HostDB (DNS resolution cache) at runtime, avoiding Traffic Server restarts when operators need to discard stale/incorrect DNS data. It introduces a restricted JSON-RPC method, wires it to a new traffic_ctl hostdb clear command, and adds a gold test to validate clearing and repopulation without restarting ATS.

Changes:

• Add restricted JSON-RPC method admin_hostdb_clear that clears HostDB’s refcounted cache under partition locks.
• Add traffic_ctl hostdb clear command that invokes the new JSON-RPC method.
• Add gold test to verify HostDB population → clear → empty → repopulation without restarting Traffic Server, plus doc updates for both JSON-RPC and traffic_ctl.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/gold_tests/dns/hostdb_clear.test.py	New gold test covering HostDB clear + repopulation behavior without restart.
src/traffic_server/RpcAdminPubHandlers.cc	Registers the new restricted JSON-RPC handler admin_hostdb_clear.
src/traffic_ctl/traffic_ctl.cc	Adds traffic_ctl hostdb clear command to the CLI parser.
src/traffic_ctl/jsonrpc/CtrlRPCRequests.h	Adds HostDBClearRequest client request mapping to admin_hostdb_clear.
src/traffic_ctl/CtrlCommands.h	Extends HostDBCommand with clear subcommand wiring.
src/traffic_ctl/CtrlCommands.cc	Implements HostDBCommand::clear() invocation via JSON-RPC.
src/mgmt/rpc/handlers/hostdb/HostDB.cc	Implements clear_hostdb() handler; updates HostDB status snapshotting to retain record handles safely outside locks.
src/iocore/hostdb/P_RefCountCache.h	Makes RefCountCache::clear() take per-partition exclusive locks before clearing.
src/iocore/hostdb/HostDB.cc	Adds HostDBProcessor::clear() that clears the underlying refcount cache.
include/mgmt/rpc/handlers/hostdb/HostDB.h	Declares the new clear_hostdb handler.
include/iocore/hostdb/HostDBProcessor.h	Declares HostDBProcessor::clear() public API.
doc/developer-guide/jsonrpc/jsonrpc-api.en.rst	Documents the new admin_hostdb_clear JSON-RPC method.
doc/appendices/command-line/traffic_ctl.en.rst	Documents traffic_ctl hostdb clear and links it to the JSON-RPC API entry.