refactor: Add clippy no panic lints

[gold-silver-copper]

Summary

• Adds workspace clippy lints for panic-prone code paths: unwrap_used, expect_used, panic, unreachable, indexing_slicing, and async lock/refcell awaits.
• Makes ProviderClient::from_env and ProviderClient::from_val return Result.
• Replaces runtime panics in provider/client/vector-store code with typed errors where practical.

Breaking Changes

• ProviderClient::from_env() now returns Result<Self, Self::Error>.
  Migration: Client::from_env() -> Client::from_env()? or explicit error handling.
• ProviderClient::from_val(...) now returns Result<Self, Self::Error>.
• Some integration constructors, including fastembed model builders, now return Result.

Notes

• Panic lints are broadly allowed in many test modules; this PR primarily hardens library/runtime code.
• Some APIs that cannot return Result, such as EmbeddingModel::make, now defer unsupported-construction failures to use time.

Verification

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