Delay missing-workspace warning until current-config fetch completes

[togisoft]

Startup could emit “Failed to discover workspace…” before linkedProjects from workspace/didChangeConfiguration was applied, even when a later fetch succeeded.
Root cause: the initial fetch ran ahead of the config-driven reload, yielding a transient empty workspace and a misleading warning.
Track a lightweight config generation and gate the warning so it only fires after a fetch completes for the current configuration.
Added config_generation and last_workspace_fetch_generation, threaded generation through fetch requests, and recorded completion.
Warning emission is now gated on fetch completion for the current config; fetch flow unchanged.
Fixes #19567.

Tests:

• Unit: missing_workspace_warning_is_gated_by_fetch_completion
• Slow: cargo test -p rust-analyzer --test slow-tests