fix(config): accept legacy invariant assert_all key

[BROCCOLO1D]

Summary

• Allows the legacy invariant.assert_all config key in warning validation for both standalone and profile-scoped invariant sections.
• Adds a regression test covering existing configs that still set assert_all.

Why

invariant.assert_all was introduced as a public config key in #12587, then later removed from the serialized invariant config when invariant campaigns became contract-level. Existing projects now see an unknown-key warning on every forge command even though the config was previously valid.

Changes

• Adds section-local backward-compatible config keys to the warning provider.
• Registers assert_all as a backward-compatible invariant key.
• Tests that [invariant] and [profile.default.invariant] usages do not emit unknown-key warnings.

Validation

• cargo fmt --check
• git diff --check
• cargo test -p foundry-config no_unknown_key_warning_for_legacy_invariant_assert_all
• cargo test -p foundry-config unknown_key_warning
• cargo check -p foundry-config

Scope

• Warning validation only; this does not reintroduce a behavior toggle for invariant campaigns.

Closes #14987

[mablr]

Thanks for your help @BROCCOLO1D.

But I'm unsure we want assert_all to be accepted as "legacy" since it lived about ten days? Moreover this doesn't address the gas snapshot problem later #14987 (comment)

[BROCCOLO1D]

Closing this based on maintainer feedback: accepting assert_all as a legacy key does not look like the right direction, and the gas snapshot regression is being handled separately in #14990. Happy to revisit if a compatibility shim is still desired.