fix: harden proxy view fee previews

[carrion256]

Summary

Stacked follow-up to PR #428 for the fee-anchor lifecycle cluster. This patch handles the latest proxy-view / fee-preview truthfulness rows with one Halborn-detectable commit per finding:

• A-085 / Nexus 86ddc60c-b4d9-4e2e-a9ad-705890b3fc89 / #FIND-085 / commit 863a2b62: include configured max_total_assets_growth_rate in the proxy_view fee snapshot tuple.
• A-077 / Nexus 082f9cb2-f33c-4006-8528-ef7a7b2918cc / #FIND-077 / commit 5842847b: replace unchecked u128 as i128 fee snapshot casts with to_i128(...) so overlarge stored fee-anchor/config values fail closed.
• A-028 / Nexus ae756926-e93c-4d54-a4a6-3a706ed20867 / #FIND-028 / commit 335a946b: compute proxy_view max_deposit / max_mint against the opposite-side conversion headroom, not only same-side total/idle/supply headroom.
• A-080 / Nexus 4460259b-12db-44b2-9396-262b531275c7 / #FIND-080 / commit 35b3685e: make fee-aware preview simulation return ConversionOverflow when management/performance fee minting would exceed u128::MAX instead of saturating supply.

Key files

• contract/vault/soroban/src/contract/entrypoints.rs
• contract/vault/soroban/src/fungible_vault.rs
• contract/vault/soroban/tests/integration_tests.rs

Verification

• cargo fmt --all -- --check
• git diff --check
• cargo test -p templar-soroban-runtime --test integration_tests soroban_contract_proxy_view -- --nocapture (4 passed)
• cargo test -p templar-soroban-runtime --test integration_tests soroban_contract_fee_aware_preview_fails_on_supply_overflow -- --nocapture (1 passed)
• cargo test -p templar-soroban-runtime --test integration_tests -- --nocapture (60 passed)
• just -f contract/vault/soroban/justfile size-budget-check (runtime deploy WASM 96709 bytes <= 131072 bytes)
• post-commit Soroban size-budget hook passed on each split commit; final head passed at 96709 bytes

Stack

Base: audit/fee-anchor-lifecycle-spec / PR #428.

---

This change is 

[coderabbitai]

Important

Review skipped

Auto reviews are disabled on base/target branches other than the default branch.

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: ac26b1d3-590e-4e1e-b5e6-b333906478b7

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch audit/fee-preview-and-proxy-view-truthfulness

---

_{Comment @coderabbitai help to get the list of available commands and usage tips.}

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: a128113c1c

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[chatgpt-codex-connector]

Preserve growth-cap absence in proxy_view

When the fee config has no max_total_assets_growth_rate (the default uncapped mode), this maps it to 0, which is indistinguishable from a configured Some(Wad::zero()). Those two states are materially different because total_assets_for_fee_accrual treats None as uncapped growth but Some(0) as no growth beyond the anchor, and governance currently accepts Some(0). Downstream consumers of the new fee snapshot cannot reconstruct the actual fee policy and may simulate fee accrual incorrectly; return a presence flag/optional encoding or another unambiguous sentinel instead of collapsing None to zero.

Useful? React with 👍 / 👎.

[carrion256]

@coderabbitai review

[coderabbitai]

✅ Actions performed

Review triggered.

Note: CodeRabbit is an incremental review system and does not re-review already reviewed commits. This command is applicable only when automatic reviews are paused.