Upgrade go.mod to Go 1.26

[sean-]

Summary

Bump the Go language version across all three modules from go 1.25.X to go 1.26. This is a prerequisite for follow-up work that depends on Go 1.26 language and standard-library features, tracked as a standalone blocking issue so dependent issues/PRs can declare it as a dependency.

This bump is deliberately deferred until after Go 1.27 is released (expected August).

Rationale & Timing

The project supports the current Go minor release and the one before it.

See prior discussion:

• Use native new(value) after the Go 1.26 bump #873 (comment) - @Jakob3xD:

  In the past we had the rule that we use one version before the current latest
  minor to not force users to update golang. Do we keep the rule? If so, using
  1.26 is only possible once 1.27 gets released in August.

  Use native new(value) after the Go 1.26 bump #873 (comment)

• Remove deprecated opensearch.ToPointer helper #881 (review) - @sean-:

  This will be reviewed in August, sometime after Go 1.27 is released and this
  project subsequently bumps its go.mod to 1.26.

  Remove deprecated opensearch.ToPointer helper #881 (review)

Background

The repository currently declares go 1.25.X in every module:

Module	Path	Current
Root client	go.mod	go 1.25.9
Code generator	cmd/osgen/go.mod	go 1.25.9
Demo	cmd/demo/go.mod	go 1.25.9

All CI workflows resolve the toolchain via go-version-file: 'go.mod', so updating the module files is sufficient to move CI to the new version - no per-workflow go-version pins need editing.

Proposed Change

Once Go 1.27 is released, update the go directive in each module:

-go 1.25.9
+go 1.26

Files to change:

• go.mod
• cmd/osgen/go.mod
• cmd/demo/go.mod