Read trigger characters from LSP initialize responses

[Firehed]

Closes #1764

Summary

ALE already reads and stores LSP-provided triggerCharacters from server capabilities, but never actually uses them. This PR wires them up so that:

• LSP trigger characters are used to decide when to automatically trigger completion
• LSP trigger characters are sent to the server in completion requests
• LSP trigger characters are used when filtering completion results
• Falls back to hardcoded s:trigger_character_map when LSP doesn't provide triggers

Changes

• Add ale#lsp#GetCompletionTriggerCharacters(conn_id) to retrieve stored trigger characters
• Add ale#lsp#GetAllCompletionTriggerCharactersForBuffer(buffer) to get triggers from all active LSPs for a buffer
• Add s:GetTriggerCharacters(filetype, conn_id) helper that prefers LSP triggers over hardcoded
• Update ale#completion#GetTriggerCharacter() to accept optional conn_id parameter
• Update ale#completion#GetPrefix() to check LSP trigger characters
• Update ale#completion#Filter() to use LSP trigger characters
• Pass connection ID through the completion flow

Test plan

• Added tests for ale#lsp#GetCompletionTriggerCharacters
• Added tests for ale#lsp#GetAllCompletionTriggerCharactersForBuffer
• Added tests for ale#completion#GetTriggerCharacter with LSP triggers
• Added tests for ale#completion#Filter with LSP triggers
• All 3359 existing tests pass
• Vint and custom linting rules pass
• Manually tested with a PHP LSP server that provides custom triggers

🤖 Generated with Claude Code

[Firehed]

I tested this out locally as well while co-developing an LSP server (which prompted me to look into this in the first place) and saw it prefer the LSP trigger characters over some hardcoded set. I use vim 9.1.1752.