…ctured not-found diagnostics

Several independent but related resolution bugs made uri+line and nodeId
lookups unreliable across almost every MCP tool:

- nodeId args were read with `.as_str()` only, so a numeric `{"nodeId": 1306}`
  (the same JSON type codegraph_symbol_search itself returns) silently failed
  to resolve. Add arg_node_id() to accept both.
- The indexer stores node paths relative to the workspace root, while
  handlers required an absolute file:// URI and compared with `==` — the two
  never matched. Worse, a failed URI parse silently became an empty string,
  which could accidentally match synthetic nodes and return an unrelated
  symbol with no error at all. Add paths_match()/resolve_uri_to_path() and
  use them everywhere a strict `==`/file:// URI was assumed.
- find_nearest_node treated nodes without an explicit line range (e.g. a
  Python module's CodeFile node) as a 0..0 range, which won every
  containment check for target_line == 0 and shadowed real symbols. Skip
  nodes without a line range instead.
- Failed resolutions returned one generic "Could not find symbol" message
  regardless of cause. Add SymbolNotFoundReason (invalid_uri /
  workspace_not_indexed / file_not_indexed) with a hint and
  suggested_next_queries(), wired into every uri+line/nodeId tool.
- Add a match_confidence ("exact" | "fallback") field, always present, so
  callers don't have to reconstruct trust level from the optional
  used_fallback field.
- codegraph_reindex_workspace could report status: "success" on a reindex
  that left the graph empty. Compare node_count after indexing against files
  processed and mark the response "degraded"; auto-retry once with
  force=true before returning, since that's what reliably fixes it.