refactor: simplify architecture of quorum observer and quorum participant

[knst]

Issue being fixed or feature implemented

It's a follow-up for #7066 and related PRs

This PR is reducing mental concentration when reading quorum-related code by significant simplification amount of levels of abstractions and inheritance classes from each other.

This PR is a direct dependency of #7234 (kernel / chainstate project)

Performance is not expected to be improved by any noticeable margin.

Compilation time is expected to be improved marginally due to fewer files (-2) to compile and less transitive includes over src/llmq/quorumsman.h.

What was done?

This changes has been spotted during #7234 and extracted to own PR.

• Remove QuorumObserverParent; decouple QuorumParticipant from QuorumObserver inheritance — they become independent implementations of a common QuorumRole interface.
• Merge QuorumObserver and ObserverContext into a single class, then remove the transitional QuorumRoleBase by folding it into QuorumRole.
• Rename src/llmq/observer/context.{h,cpp} to src/llmq/observer.{h,cpp} — flatten the now-single-file subdirectory.
• Inline QuorumParticipant into ActiveContext — it only applies to active masternode mode; src/active/quorums.{h,cpp} deleted.
• Remove four friend declarations from CDKGSession (ActiveDKGSession, ActiveDKGSessionHandler, CDKGSessionHandler, CDKGSessionManager); affected members changed from private to protected.
• Extract network code from CQuorumManager and ActiveContext into new NetQuorum (src/llmq/net_quorum.{h,cpp}) — handles QGETDATA/QDATA processing, quorum peer connections, data recovery, and periodic cleanup.

Class hierarchy — before vs. after

*by claude

Before:
QuorumObserverParent (interface, implemented by CQuorumManager)
│
QuorumObserver (base class with shared state)
├── ObserverContext (observer mode, also CValidationInterface)
└── QuorumParticipant (active mode, in src/active/quorums.h)

After:
QuorumRole (pure virtual interface in quorumsman.h)
├── ObserverContext (observer mode, in src/llmq/observer.h)
└── ActiveContext (active mode, QuorumParticipant inlined)

How Has This Been Tested?

Run unit & functional tests

Breaking Changes

N/A

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation
• I have assigned this pull request to a milestone