feat(bicep): implement signatureHelp, foldingRange, workspace/symbol locally

Bicep.LangServer 0.46.1 (latest release) does not implement these three LSP
features, so iLSP now answers them itself directly in the proxy instead of
forwarding to the underlying LS:

- textDocument/signatureHelp: curated database of ~50 commonly used Bicep
  built-in functions (resourceId, guid, concat, take, format, ...) with a
  simple bracket-depth call-context detector (ilsp/bicep_lsp/bicep_functions.py,
  ilsp/bicep_lsp/local_features.py:compute_signature_help).
- textDocument/foldingRange: stack-based bracket matcher over {}/[]/() plus
  contiguous // comment-block folding (compute_folding_ranges).
- workspace/symbol: best-effort search over documents currently open in the
  editor session (iLSP has no full-workspace index, only what's been opened),
  matching top-level resource/module/var/param/output/func/type declarations
  (compute_workspace_symbols).

The proxy now advertises these in the initialize response capabilities
(signatureHelpProvider, foldingRangeProvider, workspaceSymbolProvider) so
editors actually send the requests, and intercepts them in _client_to_ls
before they'd otherwise be silently dropped/errored by the real Bicep LS.
A send lock (_LockedConn) prevents interleaved frames now that both
_client_to_ls and _ls_to_client can write responses to the client socket.

13 new unit tests in tests/test_local_features.py. Closes the last 3 gaps
found in the full LSP capability audit.

tests/test_local_features.py

@@ -0,0 +1,132 @@
+import json
+
+from ilsp.bicep_lsp.local_features import (
+    compute_folding_ranges,
+    compute_signature_help,
+    compute_workspace_symbols,
+)
+
+
+# ── signatureHelp ────────────────────────────────────────────────────────────
+
+def test_signature_help_simple_call():
+    lines = ["var id = resourceId('Microsoft.Storage/storageAccounts', "]
+    result = compute_signature_help(lines, 0, len(lines[0]))
+    assert result is not None
+    sig = result["signatures"][0]
+    assert sig["label"].startswith("resourceId(")
+    assert result["activeParameter"] == 1
+
+
+def test_signature_help_first_arg():
+    lines = ["var g = guid("]
+    result = compute_signature_help(lines, 0, len(lines[0]))
+    assert result is not None
+    assert result["activeParameter"] == 0
+    assert result["signatures"][0]["label"].startswith("guid(")
+
+
+def test_signature_help_unknown_function_returns_none():
+    lines = ["var x = notARealFunction("]
+    result = compute_signature_help(lines, 0, len(lines[0]))
+    assert result is None
+
+
+def test_signature_help_outside_call_returns_none():
+    lines = ["var x = 'hello world'"]
+    result = compute_signature_help(lines, 0, len(lines[0]))
+    assert result is None
+
+
+def test_signature_help_nested_call_uses_innermost():
+    lines = ["var x = concat(toLower("]
+    result = compute_signature_help(lines, 0, len(lines[0]))
+    assert result is not None
+    assert result["signatures"][0]["label"].startswith("toLower(")
+
+
+# ── foldingRange ─────────────────────────────────────────────────────────────
+
+def test_folding_range_multiline_object():
+    lines = [
+        "resource sa 'Microsoft.Storage/storageAccounts@2023-01-01' = {",
+        "  name: 'demo'",
+        "  sku: {",
+        "    name: 'Standard_LRS'",
+        "  }",
+        "}",
+    ]
+    ranges = compute_folding_ranges(lines)
+    assert {"startLine": 0, "endLine": 5, "kind": "region"} in ranges
+    assert {"startLine": 2, "endLine": 4, "kind": "region"} in ranges
+
+
+def test_folding_range_ignores_single_line_brackets():
+    lines = ["var x = { a: 1 }"]
+    ranges = compute_folding_ranges(lines)
+    assert ranges == []
+
+
+def test_folding_range_comment_block():
+    lines = [
+        "// line one",
+        "// line two",
+        "// line three",
+        "var x = 1",
+    ]
+    ranges = compute_folding_ranges(lines)
+    assert {"startLine": 0, "endLine": 2, "kind": "comment"} in ranges
+
+
+def test_folding_range_short_comment_block_not_folded():
+    lines = ["// only one line", "var x = 1"]
+    ranges = compute_folding_ranges(lines)
+    assert not any(r["kind"] == "comment" for r in ranges)
+
+
+# ── workspace/symbol ─────────────────────────────────────────────────────────
+
+def test_workspace_symbol_finds_declarations():
+    docs = {
+        "file:///a.bicep": [
+            "param location string = 'westeurope'",
+            "var storageName = 'demo'",
+            "resource sa 'Microsoft.Storage/storageAccounts@2023-01-01' = {",
+            "  name: storageName",
+            "}",
+            "module roleAssignment 'br/modules:modules/roleassignments:2.0.x' = {",
+            "  name: 'ra'",
+            "}",
+        ],
+    }
+    results = compute_workspace_symbols(docs, "")
+    names = {r["name"] for r in results}
+    assert names == {"location", "storageName", "sa", "roleAssignment"}
+
+
+def test_workspace_symbol_filters_by_query():
+    docs = {
+        "file:///a.bicep": [
+            "var storageName = 'demo'",
+            "var otherThing = 'x'",
+        ],
+    }
+    results = compute_workspace_symbols(docs, "storage")
+    assert len(results) == 1
+    assert results[0]["name"] == "storageName"
+
+
+def test_workspace_symbol_query_is_case_insensitive():
+    docs = {"file:///a.bicep": ["var StorageName = 'demo'"]}
+    results = compute_workspace_symbols(docs, "storagename")
+    assert len(results) == 1
+
+
+def test_workspace_symbol_searches_across_multiple_documents():
+    docs = {
+        "file:///a.bicep": ["var fromA = 1"],
+        "file:///b.bicep": ["var fromB = 2"],
+    }
+    results = compute_workspace_symbols(docs, "")
+    names = {r["name"] for r in results}
+    assert names == {"fromA", "fromB"}