fix: integration tests NoServerForLanguage + clippy.toml msrv sync

[bug-ops]

Summary

• Fixes 12 of 19 integration tests failing with NoServerForLanguage("plaintext") (integration tests: 12 of 19 rust-analyzer tests fail with NoServerForLanguage("plaintext") #106)
• Syncs clippy.toml msrv from 1.85 to 1.88 to match Cargo.toml rust-version (clippy.toml msrv (1.85) out of sync with Cargo.toml rust-version (1.88) #107)
• Applies let-chain flattening across bridge, config, and lsp modules now valid under Rust 1.88

Changes

clippy.toml: msrv = "1.85" → msrv = "1.88"

tests/integration/rust_analyzer_tests.rs: setup_rust_analyzer() now chains .with_extensions({"rs" => "rust"}) on the test Translator. Previously Translator::new() produced an empty extension map, causing detect_language() to return "plaintext" for .rs files — the client registered under "rust" was never found.

bridge/translator.rs, config/server.rs, lsp/client.rs, lsp/lifecycle.rs, lsp/types.rs: flatten nested if let blocks into let-chains (if let A && let B { ... }), valid since Rust 1.88.

Test plan

• cargo +nightly fmt --all -- --check passes
• cargo clippy --all-targets --all-features --workspace -- -D warnings passes with no MSRV mismatch warning
• cargo nextest run --workspace --all-features --lib --bins — 344/344 pass
• RUSTDOCFLAGS="-D warnings" cargo doc --no-deps --all-features passes
• Integration tests with rust-analyzer in PATH: cargo nextest run --workspace --all-features -- --ignored integration::rust_analyzer_tests — all 19 should pass

Closes #106
Closes #107