[bug] TaskProjection: Metadata dictionary is assigned by reference, not cloned

[darrelmiller]

Problem

The current implementation of TaskProjection.Apply (and similar projection/copy methods) assigns the Metadata dictionary of AgentTask by reference, rather than cloning it. This means that mutating Metadata in a copied/returned AgentTask will also mutate the original instance. This violates the intended isolation and snapshot semantics documented in the code and PRs, and could potentially cause subtle concurrency or correctness bugs.

Context

• See discussion in PR #381
• The PR improves immutability for Artifacts and History, but not the Metadata property at the AgentTask level
• The contract and comments promise that the returned AgentTask is isolated and safe for concurrent readers, which is not currently true for Metadata

Mitigation

• Currently, Metadata is not actively mutated in projection logic, so this is not a critical bug, but represents a gap between intended design and implementation

Suggested fix

• Change TaskProjection and any similar places to clone the Metadata dictionary for new AgentTask instances (e.g., Metadata = current.Metadata is not null ? new Dictionary<string, JsonElement>(current.Metadata) : null)
• Audit other copy/project/With* implementations for the same shallow-copy issue

Impact

• Resolves the gap between doc'd isolation and actual behavior
• Avoids potential future correctness or concurrency bugs

/cc @darrelmiller (tracking so we can safely merge PR #381)