[SPRINT-02-04] Add PullPolicy and remove hidden SynBioHub fallback from local mode

[Gonza10V]

Parent sprint: #87
Depends on: #88, #89, #90
Recommended order: 4
Codex-ready: yes

Goal

Introduce an explicit pull policy so local SBOL indexing and local design resolution do not silently call SynBioHub.

Background

abstract_translator.get_or_pull(doc, sbh, uri) currently pulls from SynBioHub whenever uri is missing from the document. That behavior is convenient for live SynBioHub mode but unsafe for local/offline debugging: a local test may pass because it secretly fetched missing objects, or fail in a way that looks like an indexing bug but is actually a network/pull issue.

Scope

Add a small policy enum or constants, for example:

class PullPolicy(Enum):
    NEVER = "never"
    MISSING_ONLY = "missing_only"
    ALWAYS_REFRESH = "always_refresh"

Then update resolution helpers so callers can choose behavior explicitly:

def get_or_pull(doc, sbh, uri, pull_policy=PullPolicy.MISSING_ONLY): ...

Local indexing and local tests should use PullPolicy.NEVER.

Requirements

• PullPolicy.NEVER: return local object if present; if missing, raise or record a structured unresolved reference without network access.
• PullPolicy.MISSING_ONLY: preserve current behavior where missing objects are pulled.
• PullPolicy.ALWAYS_REFRESH: optional if easy; acceptable to add enum value and raise NotImplementedError if not needed yet, but document it.
• Update BuildCompiler local entry point to use NEVER by default.
• Keep current SynBioHub path behavior compatible by defaulting to MISSING_ONLY where appropriate.

Non-goals

• Do not implement caching or remote synchronization.
• Do not require SynBioHub credentials in local tests.
• Do not change unrelated assembly/transformation behavior.

Acceptance criteria

• Local mode has a clear way to prohibit SynBioHub pulls.
• Existing live SynBioHub path can still pull missing objects.
• Tests prove PullPolicy.NEVER does not call sbh.pull.
• Missing local references are represented clearly in an exception, audit entry, or structured warning.
• Documentation or docstrings explain when each policy should be used.

Verification

Run:

pytest -k "pull_policy or local or get_or_pull or index"
ruff check .

Codex implementation notes

• Keep the policy simple.
• Do not overfit to future architecture.
• If changing get_or_pull has many call sites, preserve default behavior and only pass NEVER from local indexing/test paths.
• Make failures precise: include the missing URI and current policy in the error/audit message.