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.

[w0rp]

I'll paste this in every open PR: We finally managed to migrate CI checks for Windows from AppVeyor to GitHub Actions, and we've updated the minimum supported Vim version to 8.2. (Thought Vim 8.0 might happen to work for some.) In order to get checks to run again for a PR you'll need to rebase or merge.

[Firehed]

Just merged an updated master - seeing CI target 8.2+ as you noted.