Clarify HostDBInfo state

[masaori335]

Summary

Replaces the "alive/dead/zombie" terminology in HostDBInfo with an explicit three-state model, "up/down/suspect", and cleans up the API surface.

HostDBInfo State

State	Description
Up	No known failure; eligible for normal selection.
Down	Blocked; no connections permitted until _last_failure + fail_window elapses.
Suspect	Fail window has elapsed; connections are permitted. On success transitions to Up; on failure returns to Down.

Changes

HostDBInfo class

• Converts struct to class with private _last_failure / _fail_count fields
• Adds State enum (UP, DOWN, SUSPECT) and a canonical state(now, fail_window) method
• Sugar helpers is_up(), is_down(now, fail_window), is_suspect(now, fail_window) for call-site clarity
• mark_down(now, fail_window) now handles both UP→DOWN and SUSPECT→DOWN transitions; the latter refreshes the fail window by updating _last_failure
• increment_fail_count takes fail_window so it can delegate to the updated mark_down
• Moves all method implementations out of the header and into HostDBInfo.cc

Documentation

• Replaces "zombie" with "suspect" throughout hostdb.en.rst

Tests

• New test_HostDBInfo.cc with unit tests covering all state transitions full UP→DOWN→SUSPECT→DOWN→SUSPECT→UP cycles

[Copilot]

Pull request overview

This PR refactors HostDB per-target health tracking by replacing the previous “alive/dead/zombie” terminology with an explicit 3-state model (UP/DOWN/SUSPECT), updating call sites and documentation, and adding unit tests around the new transition behavior.

Changes:

• Refactors HostDBInfo into a class with private failure-tracking fields and a canonical state(now, fail_window) API plus helpers.
• Updates host selection / health update call sites to use is_down(...), is_up(), and mark_active_server_up() naming.
• Adds new Catch2 unit tests and wires them into the HostDB build when BUILD_TESTING is enabled; updates HostDB developer documentation terminology.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
src/proxy/http/HttpSM.cc	Updates HostDB health update calls (mark_active_server_up, new increment_fail_count signature).
src/mgmt/rpc/handlers/hostdb/HostDB.cc	Adjusts RPC YAML encoding to use new HostDBInfo accessors.
src/iocore/hostdb/unit_tests/test_HostDBInfo.cc	Adds unit tests for UP/DOWN/SUSPECT transitions and fail window behavior.
src/iocore/hostdb/unit_tests/CMakeLists.txt	Adds a dedicated unit test executable for HostDBInfo.
src/iocore/hostdb/HostDBInfo.cc	Implements the new HostDBInfo state model and transition methods.
src/iocore/hostdb/HostDB.cc	Updates RR selection logic to use new health helpers (is_down, is_up).
src/iocore/hostdb/CMakeLists.txt	Includes the new unit_tests subdirectory under BUILD_TESTING.
include/iocore/hostdb/HostDBProcessor.h	Public API surface changes for HostDBInfo and ResolveInfo helpers; updated docs/comments.
doc/developer-guide/core-architecture/hostdb.en.rst	Replaces “zombie” terminology with “suspect” and updates behavioral description.