Fall back to prove TOML for RPC proof commands

[peter941221]

Fixes #1134

When kontrol.toml only defines proof-time settings under [prove.<profile>], the RPC proof commands that accept --config-file and --config-profile read no TOML settings at all. pyk.parse_toml_args(...) only loads [args.command], so simplify-node, step-node, section-edge, and get-model ignore the generated kontrol.toml layout and the reproducer from #1134.

This patch makes those commands load [prove.<profile>] first and then overlay any command-specific TOML section on top. Command-local overrides still win when both sections exist.

Tests cover:

• fallback from [prove.default] for all four RPC proof commands
• fallback from a selected non-default prove profile
• command-specific sections overriding the prove fallback

Validation:

• uv run make check
• uv run pytest src/tests/unit/test_toml_args.py -q
• uv run make test still fails on a clean master clone with the same existing pytest-xdist collection mismatch in src/tests/unit/test_natspec.py, so this PR does not introduce that failure

[peter941221]

@palinatolmach I tightened the regression coverage for #1134 and marked this ready for review.

Validation on this branch:

• uv run make check
• uv run pytest src/tests/unit/test_toml_args.py -q

I also checked uv run make test against a clean master clone. It hits the same existing pytest-xdist collection mismatch in src/tests/unit/test_natspec.py, so that failure is not introduced by this PR.