Fix remaining uninitialized variable and field Coverity defects

[bryancall]

Summary

Add in-class brace initialization for uninitialized members in
HttpVCTableEntry, HostStatRec, ChunkedHandler, and TLSSNISupport
ClientHello. Initialize local enum and Feature variables in
cache_fill and txn_box plugins.

CIDs: 1021690, 1508845, 1533658, 1534712, 1544456, 1645800.

Changes

CID	File	Change
1533658	cache_fill/configs.cc	Initialize local OP enum in readConfig
1534712	txn_box/Ex_Base.cc	Feature zret{} → Feature zret{NIL_FEATURE}
1021690	HttpTunnel.h	max_chunk_size → {DEFAULT_MAX_CHUNK_SIZE}
1508845	HttpVCTable.h	Brace-initialize all HttpVCTableEntry members
1544456	HostStatus.h	Brace-initialize all HostStatRec members
1645800	TLSSNISupport.h	_ext_len → {0} in ClientHello

Test plan

• CI passes (no behavioral changes, initialization only)
• Verify Coverity scan marks target CIDs as fixed

[Copilot]

Pull request overview

This PR addresses a set of Coverity “uninitialized variable / field” defects by adding deterministic default initialization for several structs and locals across core proxy code and experimental plugins.

Changes:

• Add in-class member initialization for HttpVCTableEntry, HostStatRec, ChunkedHandler::max_chunk_size, and TLSSNISupport::ClientHello::_ext_len.
• Initialize previously uninitialized locals in experimental plugins (txn_box Feature return value, cache_fill enum op).

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
plugins/experimental/txn_box/plugin/src/Ex_Base.cc	Initialize Feature return value to NIL_FEATURE to avoid default/uninitialized state.
plugins/experimental/cache_fill/configs.cc	Initialize Content-Length comparison operator enum before parsing.
include/proxy/http/HttpVCTable.h	Add in-class initialization for all HttpVCTableEntry members.
include/proxy/http/HttpTunnel.h	Default-initialize ChunkedHandler::max_chunk_size.
include/proxy/HostStatus.h	Default-initialize HostStatRec fields to a known state.
include/iocore/net/TLSSNISupport.h	Default-initialize ClientHello::_ext_len to 0.