refactor(quickadapter): consolidate Optuna sampler tuples to NamedTuple (#101)

refactor(quickadapter): consolidate Optuna sampler tuples to NamedTuple (#101)

PR #81 consolidated `_OPTUNA_NAMESPACES` to `Utils._OptunaNamespaces` (a `NamedTuple` with per-field singleton `Literal` types). Propagate the same pattern to the three sibling sampler tuples in `QuickAdapterRegressorV3.py`:

- `_OPTUNA_SAMPLERS` (tpe, auto, nsgaii, nsgaiii)
- `_OPTUNA_HPO_SAMPLERS` (tpe, auto)
- `_OPTUNA_LABEL_SAMPLERS` (auto, tpe, nsgaii, nsgaiii)

Each new `_Optuna*Samplers` NamedTuple class lives at module level immediately before `class QuickAdapterRegressorV3`, matching the `_OptunaNamespaces` adjacency in `Utils.py`. Per-field types are singleton `Literal["..."]` (not the `OptunaSampler` union) to unlock pyright/mypy narrowing for a future `assert_never` migration. The class-private `_OPTUNA_*_SAMPLERS` instance constants remain class-attributes of `QuickAdapterRegressorV3` to preserve every consumer's `QuickAdapterRegressorV3._OPTUNA_*` access pattern.

Each `_Optuna*Samplers` redeclares its field defaults (per-class `Literal["tpe"] = "tpe"` etc.), replacing the prior tuple-slice derivation `_OPTUNA_HPO_SAMPLERS = _OPTUNA_SAMPLERS[:2]` and the 6-line custom reordering of `_OPTUNA_LABEL_SAMPLERS`. Trade-off: minor string-literal duplication across 3 classes is accepted for harmonization with the `_OptunaNamespaces` template; the single source of truth for the 4 valid tokens stays the `OptunaSampler = Literal[...]` alias unchanged at module level.

Migrates 8 positional-indexing call sites (`_OPTUNA_*_SAMPLERS[N]  # "name"`) to attribute access (`.<name>`); drops the 6 surviving inline annotations at the call sites (2 sites carry no annotation pre-refactor; the multi-line site collapses 3 source lines into 1) plus the 4 analogous annotations inside the deleted 6-line `_OPTUNA_LABEL_SAMPLERS` custom-ordering block. The `_OPTUNA_HPO_SAMPLERS_SET` and `_OPTUNA_LABEL_SAMPLERS_SET` frozenset companions are kept unchanged (still used in O(1) membership testing at `optuna_samplers_by_namespace`); their type annotation `Final[frozenset[OptunaSampler]]` is preserved.

Non-migration sites confirmed unchanged: the frozenset companion construction, the `', '.join` error-message iteration over `_OPTUNA_SAMPLERS`, and the `_SET` membership references. All iterate over the NamedTuple instance and produce byte-identical output.

Add `NamedTuple` to the existing `from typing import (...)` block (alphabetical, between `Literal` and `Optional`). `assert_never` already imported, kept in anticipation of the deferred follow-up.

Per-field singleton `Literal[...]` unlocks `assert_never` exhaustiveness on the `optuna_create_sampler` dispatch chain -- left to a follow-up PR since the migration (`else: raise ValueError(...)` becoming `assert_never(sampler)`) changes the user-facing error contract on the unreachable branch.

The AGENTS.md *Canonical defaults* principle and the README documented enum order are encoded in the field-declaration order of each `_Optuna*Samplers`. NamedTuple remains a `tuple` subclass, so `[0]` indexing, `len(...)`, `frozenset(...)`, `', '.join(...)`, and iteration all keep their existing semantics. No behavior change.

Reviewed by two pre-implementation design passes (3-oracle on v1; Metis + Momus + meta-Oracle on v2) and a 3-oracle review on the live PR, each citing upstream evidence from `freqtrade/freqai/` confirming no external consumer.

Follow-up from PR #81 review (Oracle harmonization dimension).

Closes #88.