[Bug]: LLM and pydantic selectors silently route to the last choice on 0-indexed answers

Bug Description

Both selector paths in llama_index.core.selectors ask the model to answer with 1-based choice numbers (_build_choices_text enumerates choices as (1), (2), ...), then subtract 1 from the model's answer without validating it:

• _structured_output_to_selector_result (llm_selectors.py): SingleSelection(index=answer.choice - 1, ...)
• _pydantic_output_to_selector_result (pydantic_selectors.py): output.index -= 1

Models do sometimes answer with 0-indexed or over-range numbers. When that happens:

• choice = 0 becomes index = -1, and Python's negative indexing silently selects the last choice. In RouterQueryEngine this is self._query_engines[result.ind], so the query goes to the wrong engine with no error. The same silent misrouting applies anywhere SelectorResult.ind / inds is used to index into the choice list.
• An over-range choice (e.g. choice = 5 with 3 choices) passes through and crashes downstream with an unhelpful IndexError: list index out of range.

This is the same failure mode that #22827 fixed for StructuredLLMRerank (a document_number of 0 mapping to nodes_batch[-1]); the selectors never got the same guard.

Proposed fix, mirroring #22827: validate selections against the number of choices, drop out-of-range entries, and raise a clear ValueError when nothing valid remains (a single selector cannot return an empty result). I have the patch and regression tests ready - happy to send the PR.

Version

llama-index-core 0.14.24 (also verified on current main, fd4a517)

Steps to Reproduce

from llama_index.core.selectors.pydantic_selectors import _pydantic_output_to_selector_result
from llama_index.core.selectors.llm_selectors import _structured_output_to_selector_result
from llama_index.core.base.base_selector import SingleSelection
from llama_index.core.output_parsers.base import StructuredOutput
from llama_index.core.output_parsers.selection import Answer

# A 0-indexed answer (model meant the FIRST choice)
r = _pydantic_output_to_selector_result(SingleSelection(index=0, reason="first"), 3)
print(r.selections[0].index)  # -1

engines = ["engine_a", "engine_b", "engine_c"]
print(engines[r.selections[0].index])  # engine_c - silently the LAST engine

r2 = _structured_output_to_selector_result(
    StructuredOutput(raw_output="", parsed_output=[Answer(choice=0, reason="first")]), 3
)
print(r2.selections[0].index)  # -1

# Over-range passes through, then crashes at the indexing site
r3 = _pydantic_output_to_selector_result(SingleSelection(index=99, reason="over"), 3)
print(engines[r3.selections[0].index])  # IndexError

Output:

-1
engine_c - silently the LAST engine
-1
Traceback (most recent call last):
  ...
IndexError: list index out of range