Fix PHP symbol retrieval: declare missing Intelephense client capabilities

[EarthmanWeb]

Problem

Intelephense's LSP server conditionally enables certain features based
on what the client declares it supports in the initialize request.
Serena's Intelephense client was missing several capability declarations:

• textDocument.documentSymbol (with hierarchicalDocumentSymbolSupport)
• textDocument.references
• textDocument.hover
• workspace.symbol

Without hierarchicalDocumentSymbolSupport: true, Intelephense falls
back to returning flat SymbolInformation[] instead of the nested
DocumentSymbol[] tree. This means symbol retrieval works partially
or not at all — classes and their methods appear as disconnected
root-level symbols with no parent-child relationship.

Additionally, when find_symbol is called with a specific PHP file
path, it iterated all language servers rather than routing to the
correct one, causing PHP files to be rejected by non-PHP servers.

Fix

• intelephense.py: Added missing client capability declarations
  for documentSymbol (with full hierarchicalDocumentSymbolSupport
  and symbolKind range), references, hover, and workspace
  symbol. Also added assertion that the server confirms
  documentSymbolProvider support after initialization.

• symbol.py: When find_symbols is called with a specific file
  path, route to the appropriate language server via
  get_language_server() instead of iterating all servers.

Example

Before:

# Intelephense returned flat SymbolInformation[] — no hierarchy
symbols = get_symbols_overview("src/Controller.php")
# → {"MyController": [], "index": [], "show": []}
# (methods appear at root level, not nested under class)

After:

symbols = get_symbols_overview("src/Controller.php")
# → {"MyController": {"index": {}, "show": {}}}
# (proper hierarchy via DocumentSymbol[])

[opcode81]

If it shall provide the basis for tests (which it probably should), please move this the PHP test repository.
If it shall not provide the basis for tests, delete it.

[opcode81]

Tests should be added which validate that the changes result in the improvements described.
In particular, we should explicitly inspect the hierarchical structure in tests.