feat: merge variables from required input packages into composable manifests

[teresaromero]

Merge input package variables into composable integration manifests

Closes elastic#3279

Composable packages stack — part 2 of 4.
This PR is stacked on elastic#3329 (bundle templates, closes elastic#3278). Merge order: elastic#3329 → this PR → #2 → #3.
Each PR will be rebased onto main once its predecessor merges.

Summary

Extends the required-inputs build step (introduced in elastic#3329) to merge variable definitions from required input packages into the composable integration's policy template and data stream manifests.

Variable merging follows these rules:

• Input package variables are the base.
• The integration package can promote input-level variables by declaring them under policy_templates[].inputs[].vars — they surface as top-level input vars and can override title/description/default.
• Remaining variables land at data-stream level; the integration can override individual field-level properties without redeclaring the whole variable.
• If the same variable appears at both levels in the integration manifest, the build fails with a clear error.

What changed

internal/requiredinputs/variables.go (new, ~500 lines)

• MergeVariables: entry point called after template bundling.
• Handles variable promotion to input level vs placement at data-stream level.
• Merge priority: integration-declared override fields win; unset fields fall through to input package defaults.

internal/requiredinputs/variables_test.go (new, ~420 lines)

• Covers: full merge, DS-only merge, input promotion, no-override passthrough, duplicate-variable error, two-policy-template scoping.

test/manual_packages/required_inputs/ (new fixtures)

• var_merging_input_pkg — input package defining base variables.
• with_merging_full — promotes paths+encoding; DS merges timeout; adds custom_tag.
• with_merging_promotes_to_input, with_merging_ds_merges, with_merging_no_override — targeted scenarios.
• with_merging_duplicate_error — expects build failure.
• with_merging_two_policy_templates — validates per-policy-template scoping.

test/manual_packages/README.md — documents manual test fixture usage.

How to test

# Build the variable-merging input package
cd test/manual_packages/required_inputs/var_merging_input_pkg
elastic-package build

# Start the stack (serves the built package via built-in registry)
elastic-package stack up --version 9.4.0-SNAPSHOT

# Build and inspect a merge scenario
cd test/manual_packages/required_inputs/with_merging_full
elastic-package build
# Inspect build/ — merged variables should appear in policy_templates and data_stream manifests

Dependencies

• bundle templates of required input packages elastic/elastic-package#3329 must merge first.