fix(modal): reuse env secrets across sandbox operations

[EazyReal]

Problem

The Modal environment injects persistent task env and per-exec env through Secret.from_dict. Under high concurrency, identical env payloads can produce one ephemeral Modal Secret per sandbox startup and per exec. Modal applies workspace-wide secret create limits, so a startup or setup burst can fail with transient infrastructure errors even though the env payload is unchanged.

Approach

• Cache Secret.from_dict objects by an order-independent env payload key.
• Reuse that cached env secret for both sandbox creation (_secrets_config) and per-command execution (_sdk_exec).
• Keep named Modal secrets unchanged.
• Increase sandbox creation retries from 2 to 4 attempts with a slightly longer exponential backoff cap for residual transient provider spikes.
• Add regression coverage for same-payload reuse, distinct-payload separation, persistent sandbox env injection, and per-exec env injection.

Validation

• uv run pytest tests/unit/environments/test_modal.py -q -> 28 passed
• uv run ruff check --fix . -> All checks passed!
• uv run ruff format . -> 1004 files left unchanged
• uv run ty check -> All checks passed!
• uv run pytest tests/unit -q -> 2723 passed, 1 skipped

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[EazyReal]

Superseded by #1806 so the source branch can use modal-secret-cache instead of codex/modal-secret-cache.