Read trigger characters from LSP initialize responses (#5121)

* Add ale#lsp#GetCompletionTriggerCharacters getter
* Add s:GetTriggerCharacters helper with LSP support
* Pass connection ID to GetTriggerCharacter in s:OnReady
* Use LSP trigger characters in Filter function
* Add tests for LSP completion trigger characters
* Check LSP trigger characters in GetPrefix
* Add tests for GetAllCompletionTriggerCharactersForBuffer

GetTriggerCharacter now accepts optional conn_id parameter to
prefer LSP-provided trigger characters over the hardcoded map.

GetPrefix now checks if the line ends with any trigger character
from LSPs active for the current buffer, enabling automatic
completion for LSP-provided triggers like > for PHP.

test/lsp/test_other_initialize_message_handling.vader

@@ -343,3 +343,51 @@ Execute(Results that are not dictionaries should be handled correctly):
   \ 'result': v:null,
   \})
   AssertEqual [], g:message_list
+
+Execute(GetCompletionTriggerCharacters should return stored characters):
+  call ale#lsp#HandleInitResponse(b:conn, {
+  \ 'jsonrpc': '2.0',
+  \ 'id': 1,
+  \ 'result': {
+  \   'capabilities': {
+  \     'completionProvider': {
+  \       'triggerCharacters': ['@', '#', '.'],
+  \     },
+  \   },
+  \ },
+  \})
+
+  AssertEqual ['@', '#', '.'], ale#lsp#GetCompletionTriggerCharacters(b:conn.id)
+
+Execute(GetCompletionTriggerCharacters should return empty for missing connection):
+  AssertEqual [], ale#lsp#GetCompletionTriggerCharacters('nonexistent-connection')
+
+Execute(GetCompletionTriggerCharacters should return empty when no triggers):
+  call ale#lsp#HandleInitResponse(b:conn, {
+  \ 'jsonrpc': '2.0',
+  \ 'id': 1,
+  \ 'result': {
+  \   'capabilities': {},
+  \ },
+  \})
+
+  AssertEqual [], ale#lsp#GetCompletionTriggerCharacters(b:conn.id)
+
+Execute(GetAllCompletionTriggerCharactersForBuffer should return triggers for open buffers):
+  call ale#lsp#HandleInitResponse(b:conn, {
+  \ 'jsonrpc': '2.0',
+  \ 'id': 1,
+  \ 'result': {
+  \   'capabilities': {
+  \     'completionProvider': {
+  \       'triggerCharacters': ['>', '$'],
+  \     },
+  \   },
+  \ },
+  \})
+
+  call ale#lsp#MarkDocumentAsOpen(b:conn.id, 1)
+  AssertEqual sort(['>', '$']), sort(ale#lsp#GetAllCompletionTriggerCharactersForBuffer(1))
+
+Execute(GetAllCompletionTriggerCharactersForBuffer should return empty for unknown buffer):
+  AssertEqual [], ale#lsp#GetAllCompletionTriggerCharactersForBuffer(99999)