feat(pricing): make DeepSeek V4 Pro 75% discount permanent

[Colorful-glassblock]

Summary

Removed the V4 Pro discount expiry logic in the TUI cost estimator so the 75% discount is treated as permanent, matching the updated official pricing.

Testing

• cargo test --all-features
• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features

Checklist

• Updated docs or comments as needed
• Added or updated tests where relevant
• Verified TUI behavior manually if UI changes

[gemini-code-assist]

Code Review

This pull request updates the README.md to reflect that the 75% discount for DeepSeek Pro is now permanent. Feedback indicates that the TUI's internal pricing logic and associated tests in crates/tui/src/pricing.rs must be updated to remove the expiration logic, and localized README files should be synchronized. Additionally, a suggestion was made to use a language-neutral URL for the pricing link in the English documentation.

[gemini-code-assist]

The documentation update indicates that the 75% discount is now permanent. However, the TUI's cost calculation logic in crates/tui/src/pricing.rs still relies on v4_pro_discount_ends_at() (set to 2026-05-31) to determine when to revert to base rates. To prevent the TUI from displaying incorrect prices after that date, the logic in pricing.rs and its associated tests (e.g., v4_pro_returns_to_base_rates_after_discount_expiry) should be updated. Additionally, README.zh-CN.md and README.ja-JP.md should be synchronized with these changes to maintain consistency across all localized documentation.

[gemini-code-assist]

In the English README, it is preferable to link to the language-neutral or English version of the pricing page rather than the zh-cn specific one.

Suggested change

	> DeepSeek-V4-Pro was previously offered at a 75% discount originally scheduled to end at 15:59 UTC on 31 May 2026. This discount has now been made permanent. For the latest pricing, please consult the official [DeepSeek pricing page](https://api-docs.deepseek.com/zh-cn/quick_start/pricing). All rates listed in the README correspond to the officially published values.
	> DeepSeek-V4-Pro was previously offered at a 75% discount originally scheduled to end at 15:59 UTC on 31 May 2026. This discount has now been made permanent. For the latest pricing, please consult the official [DeepSeek pricing page](https://api-docs.deepseek.com/quick_start/pricing). All rates listed in the README correspond to the officially published values.

[Colorful-glassblock]

Windows test failure (cache_inspect_reports_tool_result_budget_metadata) is a pre-existing flaky test unrelated to this PR. The failure is in crates/tui/src/client/chat.rs:3000 — the dedup ref exceeded the 300-char budget because SHA spillover persistence likely failed on the Windows runner, causing the compactor to inline the full 14k-char tool result. This PR only touches crates/tui/src/pricing.rs and docs.

[weikengchen]

I think it has overlapping with #1932 on the README side, but the TUI cost estimator needs to remove the end-time thing.

[Colorful-glassblock]

@weikengchen Thanks for the heads-up! I'll drop the README changes from this PR to avoid overlapping with #1932. Appreciate you catching that — this PR will focus solely on the pricing.rs TUI cost estimator change (removing the discount end-time logic).

[carloslindhstrom-code]

crates/tui/src/pricing.rs

[Hmbown]

Thanks for tightening the scope after the README overlap was pointed out. I am not merging pricing changes in v0.8.42 without a fresh check against the official DeepSeek pricing source on the exact day we land it.

For the next pass, please keep this PR limited to crates/tui/src/pricing.rs plus tests around the cost estimator. Maintainers should verify current official pricing first, then update docs/changelog from that source rather than relying on issue/PR text.

[Colorful-glassblock]