feat: the binder bridges a collection of collections

[ShocOne]

Cut from main after #166. Second of the three nested-collection PRs; the emit + fixtures + curated-fixture PR follows once this merges.

What

The SDK binding layer and the conversion catalog carry list of lists, list of maps, map of lists and map of maps — and deeper, with a scalar at the bottom — under both dialects. Nothing generates differently yet: derivation still excludes depth ≥ 2 under nestedCollectionElement until the emit PR removes that gate, so this lands the machinery with its tests and changes no generated tree.

Binder (internal/sdkbind)

• FieldBinding.NestedCollectionElementTypes copies the derivation's levels.
• Both binders draft one plain SDK type composed to depth (nestedCollectionGoType: [][]string, map[string]map[string]int64) and one conversion pair, FromNestedCollectionSlice/Map / ToNestedCollectionSlice/Map.
• prune_collections.go: settleCollection settles the draft against the carrier the SDK actually has, in order:
  1. kiota untyped node (serialization.UntypedNodeable, matched on package path like isKiotaDateOnly) → …SliceUntypedNode; a map declared over one is excluded.
  2. one bag model (GetAdditionalData) for a map of lists / map of maps → …MapAdditionalData, model + write constructor carried (the existing scalar bag arm now shares settleBag).
  3. slice of bag models for a list of maps → …SliceAdditionalData.
  4. plain Go value matched level by level (plainCollectionMatches: slice ↔ list, string-keyed map ↔ map, compatible basic leaf; a pointer at any level, a named leaf or a non-string key does not match) → …Slice/Map.
  5. otherwise unbridgeableType, the reason naming the declared shape ("does not nest the way the document declares a map of list of string").
• nestedModelOf unwraps every collection level, so an object at the bottom of a nested collection resolves against its model.

Conversion catalog (templates)

• Shared: APIToFrameworkNestedCollectionList/Map(ctx, values any, elementType attr.Type) and FrameworkToAPINestedCollectionList/Map[T any](ctx, value, setter func(T)) — the framework's own reflection walks nested Go values; nil at any depth is null, empty stays empty.
• nested_collections_kiota.go.tmpl: …MapAdditionalData, …ListAdditionalData[T] (one model per element, built through a construct func() T), …ListUntypedNode; walkers frameworkValueFromBag / bagValueFromFramework / anyFromUntypedNode / untypedNodeFromAny. Rules: a parsed bag holds []any, map[string]any and pointer scalars with every number a float64; a stray map entry is skipped alone (as test: the curated fixture carries a kiota additionalData bag #162 decided), a stray list element becomes the typed null so positions hold; nil is the typed null at every level.
• bagString / bagBool extracted so the four scalar bag helpers and the walkers share one leaf rule.

SDK shapes this was verified against

Generated from a probe spec with kiota 1.34.1 and openapi-generator (scratchpad, not committed): openapi-generator emits [][]string, []map[string]string, map[string][]string, map[string]map[string]string; kiota emits UntypedNodeable for a list of lists, []Thing_listOfMapsable for a list of maps, and a single Thing_mapOfLists/Thing_mapOfMaps bag model for the map forms. GetRawValue in kiota-serialization-json-go@v1.1.4 unwraps nested arrays/objects recursively; WriteAdditionalData marshals []any/map[string]any through json.Marshal.

Tests

• prune_collections_test.go: every pairing at two levels and one at three through the plain arm; seven shapes that must not match (short, deep, map-for-list, pointer, non-string key, named leaf, incompatible leaf); map of lists through a bag; list of bags with the constructor; list of lists through the untyped node (and a foreign UntypedNodeable not matching); nestedModelOf to depth; nestedCollectionGoType / describeCollection.
• Binder drafting tests in both dialect test files.
• Template tests run inside both generated curated trees by TestUnit_Run_CuratedTreeCompiles: plain round trips for all four shapes, nil/empty/null/unknown rules; kiota bag forms, stray-entry vs stray-element rules, bag round trip, one-model-per-element, untyped-node round trip and a scalar node reading as null.

Verification

make check green: sdkbind 83.3% (from 82.6%), 90.2% total, hygiene gate clean.

🤖 Generated with Claude Code

https://claude.ai/code/session_01XPLePCqzyYncF7TedagFkg