feat: add overriden and extended resolver support

[dgandhi62]

How extended resolvers work

In Gen1, extended resolvers are VTL files with 6-segment filenames like Mutation.createBoard.init.2.req.vtl. The segments encode the target resolver (Mutation.createBoard), the pipeline slot (init), the execution order within that slot (2), and the template type (req). Gen1 places these as separate pipeline functions at the appropriate slot position.

In Gen2, we recreate this by generating AppsyncFunction constructs with a NoneDataSource and splicing them into the existing pipeline resolver's function array at the correct position.

Type-specific slot mapping

Different GraphQL operation types have different valid slots and different default pipeline structures:

• Query: [auth0, postAuth0, DataResolverFn] — slots include preDataLoad/postDataLoad
• Create/Update Mutation: [init0, auth0, postAuth0, DataResolverFn] — slots include preUpdate/postUpdate
• Delete Mutation: [auth0, postAuth0, DataResolverFn] — same slots as create/update but shorter default pipeline
• Subscription: [auth0, postAuth0, DataResolverFn] — slots include preSubscribe

The splice index computation accounts for these differences by selecting the correct base index mapping based on the operation type and field name prefix.

Override resolver filter fix

The generated override resolver loop previously matched all .req.vtl/.res.vtl files regardless of segment count. This caused 6-segment extended resolver files to be incorrectly applied as DataResolverFn template overrides. The filter now includes a f.split(".").length === 4 guard to only process 4-segment override files.

Splice index computation algorithm

The algorithm places custom functions into the Gen2 pipeline at positions that match their Gen1 slot behavior. It works in three steps:

1. Determine the default pipeline shape. The base indexes depend on which default functions exist:

Create/Update Mutation: [init0(0), auth0(1), postAuth0(2), DataResolverFn(3)]
Query / Delete Mutation / Subscription: [auth0(0), postAuth0(1), DataResolverFn(2)]

Each slot maps to a base index — the position after the last default function for that slot:

                        With init0    Without init0
init:                      1              0
preAuth:                   1              0
auth:                      2              1
postAuth:                  3              2
preDataLoad/preUpdate:     3              2
postDataLoad/postUpdate:   4              3
finish:                    4              3

2. Sort custom functions. Functions are sorted by slot (pipeline execution order), then by numeric order within the same slot. The order number in the filename only determines relative ordering among custom functions in the same slot — it does not map to an absolute pipeline position.

3. Compute splice indexes with a running offset. Walk through the sorted functions in order. For each function:

spliceIndex = baseIndex[slot] + offset
offset += 1

The offset accounts for prior insertions shifting subsequent positions. Example with init.2 and finish.1 on a create mutation:

Start:    [init0, auth0, postAuth0, DataResolverFn]

init.2  → base=1, offset=0 → splice(1)
Result:   [init0, init2, auth0, postAuth0, DataResolverFn]

finish.1 → base=4, offset=1 → splice(5)
Result:   [init0, init2, auth0, postAuth0, DataResolverFn, finish1]

Checklist

• PR description included
• yarn test passes
• Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)
• New AWS SDK calls or CloudFormation actions have been added to relevant test and service IAM policies
• Pull request labels are added

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