feat!: remove easyjson from go-sdk-common

[aaron-zeisler]

Summary

This is the v4 major-version bump for go-sdk-common. Two commits:

1. chore: remove easyjson implementation — drops the optional easyjson backend, removes the easyjson conversion files for ldcontext / ldreason / ldvalue, and bumps go-jsonstream to a development pseudo-version that has its own easyjson removal merged.
2. feat!: bump module path to v4 — bumps the module path /v3 → /v4, rewrites all internal imports, bumps go-jsonstream from /v3 to /v4. Workflow file changes for v4 landed separately in #52.

SDK-2119: EasyJSON Removal — Dependency Chain (v4 strategy)

go-jsonstream/v4 (#39)                  <- feat!:
└─ go-sdk-common/v4 (this PR)           <- feat!:
   ├─ go-server-sdk-evaluation/v4 (#51) <- feat!:
   ├─ go-sdk-events/v3 (#43)            <- chore:
   └─ go-server-sdk/v7 (#380)           <- chore:
      └─ ld-relay (#666)                <- chore:
         └─ internal services (6 PRs)   <- chore:

Stage layout

Stage	Purpose	PRs
1	Configure v4 branch's CI workflows	#40, #52, #53
3	v4 major bumps + EasyJSON removal (libraries)	#39, #50, #51
4	Cascade /v3 → /v4 through 10 downstream PRs (dev pseudo-versions)	#43, #380, #666, #8924, 6 services
5	After v4.0.0 tags land, flip downstream go.mod from pseudo-versions to real	(post-merge)

Context

Part of SDK-2113 (epic). Tracks SDK-2117. Second PR in the dep-chain stack — pins go-jsonstream/v4 at #39 and is itself pinned by #51.

Test plan

• go test ./... passes (v4 module path)
• go build ./... clean
• go mod graph | grep easyjson returns empty
• grep -r mailru/easyjson . returns no source hits

via LD Research 🤖

---

Note

High Risk
Breaking major version with module path and JSON stack changes affects all dependent SDKs and services; incorrect migration breaks builds and context/value serialization at runtime.

Overview
This PR ships go-sdk-common v4 as a breaking release aligned with Go SDK 7.x: the module path moves from /v3 to /v4, Go 1.24 is required, and go-jsonstream is upgraded to v4 with imports updated repo-wide.

The functional change is removal of the optional EasyJSON integration: mailru/easyjson is dropped from dependencies, all //go:build launchdarkly_easyjson sources (context, ldvalue, ldreason marshaling/tests/benchmarks) are deleted, and JSON handling stays on encoding/json plus jsonstream only. Docs no longer describe the launchdarkly_easyjson build tag.

Downstream consumers must import github.com/launchdarkly/go-sdk-common/v4 and cannot rely on EasyJSON-specific APIs from this library.

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