Fix PHP find_symbol: handle flat SymbolInformation

[EarthmanWeb]

Problem

Two related issues cause find_symbol to fail or return wrong results in PHP projects:

1. Incorrect language server routing
In multi-language projects, get_language_server() routed files using
is_ignored_path() — a method that checks gitignore and unsupported
extension filters. A PHP file could pass or fail this check for the
wrong language server depending on configuration order, causing PHP
files to be routed to the Python LS (or vice versa).

2. Flat SymbolInformation[] from Intelephense
For some PHP files, Intelephense returns SymbolInformation[] (the
older LSP format) instead of DocumentSymbol[]. These symbols have no
parent-child hierarchy — methods appear as root-level symbols with only
a containerName string field linking them to their class.

find_symbol("ClassName/method") failed silently because
iter_name_path_components_reversed() never yielded the container,
so the name-path match never succeeded.

3. Hard error on explicitly-requested ignored file
When get_symbols_overview() was called on a file that happened to
match an ignore pattern, it raised a hard error and returned []
instead of proceeding. This broke workflows where the caller explicitly
requested a specific file.

Fix

• ls_manager.py: Route by fn_matcher.is_relevant_filename() —
  checks the file extension against the language's known extensions —
  instead of is_ignored_path().

• symbol.py: Added containerName fallback in
  iter_name_path_components_reversed(). When a symbol has no
  ancestors (flat SymbolInformation), yield the containerName value
  so name-path patterns like "ClassName/method" still match.

• ls.py: Downgraded hard error to a warning when an explicitly
  requested file is ignored; proceed with the symbol request anyway.

Example

Before:

# In a Python+PHP project:
find_symbol("TestWebhookHandler/handle_request")
# → [] (routed to wrong LS, or containerName not traversed)

After:

find_symbol("TestWebhookHandler/handle_request")
# → [<LanguageServerSymbol: handle_request>]

[MischaPanch]

@opcode81 @EarthmanWeb what's the status here?

[EarthmanWeb]

Rebased on current main and added the cache invalidation fix.

cache_version_raw_document_symbols is now set to 2 in Intelephense's super().init call, since _request_document_symbols now reconstructs flat SymbolInformation[] into DocumentSymbol[], changing the cached data format.

The get_language_server routing change was dropped — upstream's _get_suitable_language_server already handles extension-based routing correctly.