[roll] Roll infra/recipes [build] Re(5)-land: Migrate build recipe to main_build.py

Reland of commit bd61a2da6166ee6715c5bafb1672ea2dd480671a
(reverted in 4ad6d1d715c9009d087f3b19f745edf5f1c31ca1).

This change removes much logic around remote service setup for
driving Fuchsia builds, now relying on main_build.py as the
singular entry point shared by infra and developer (fx) builds.

recipe_modules/build/api.py:
- Resolves main_build.py dynamically from tool_paths.json.
- fint-build wrapping is now an internal detail of main_build.py,
  but we use output_metadata.json to get paths to important build
  artifacts and logs. No longer use the (Go) 'fint build'.
  - Adds parsing of output-metadata-json to resolve open logs,
    RBE diagnostics, and detailed rrpl action logs for CAS uploading.
- Streamlines recipe-side authentication by passing
  main_build.py --auth-mode=machine, delegating credentials handling
  to main_build.py.
- Eliminates redundant RBE and ResultStore environment overrides on
  the recipe side, now passing key service sockets as main_build.py
  --remote-proxy-socket and --resultstore-proxy-socket (for
  ResultStore).
- Wraps sandbox execution in a Path-casted context manager
  (cwd=context.checkout_dir) to ensure robust relative path resolution
  inside nsjail.
- Factored out _read_output_metadata, _process_rbe_metadata, and
  _nsjail_sandbox_cmd helpers.

Fixes since Re(4)-land was reverted:
* Plumb --remote-proxy-socket whenever ResultStore is enabled, even if
  RBE compilation is disabled (e.g. no_rbe builders). This ensures
  rsproxy receives a valid RS_cas_service endpoint matching the old recipe
  behavior and avoids a deadlock in rsproxy's file-upload pipeline when
  uploading stdout/stderr of actions with CAS disabled.

Follow-ups:
- Delete resultstore recipe_module.
- We cannot delete the rbe module because some non-Fuchsia builders
  require it for reproxy setup/teardown.

Original-Original-Bug: 450234102, 481021388, 570223323
Original-Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/infra/recipes/+/1874076
Original-Original-Revision: f8eb377a7c5b553ba0463b2582a4cb1e033316ef
GitOrigin-RevId: 65a64628f60b1172cee1d81a3acdc036caf0badb
Change-Id: Id682f75f95eeef236e86ad4e61a4ef58ab13c4c4
Reviewed-on: https://fuchsia-review.googlesource.com/c/integration/+/1875047
Cr-Commit-Position: refs/heads/main@{#204723}