Fix Invalid RUST_LOG

[JSE19]

Description

• What changed: Updated init_tracing() in oracle/src/main.rs to distinguish between an unset RUST_LOG environment variable and a malformed one. If RUST_LOG is provided but fails to parse, it falls back to "info" verbosity and emits a tracing::warn! immediately after initializing tracing, naming the invalid value and the parser error.
• Why: Previously, EnvFilter::try_from_default_env() collapsed missing RUST_LOG and malformed values into a silent fallback to "info". An operator with a typo in their log filter received no feedback that their directive was rejected, leaving no diagnostic trail.
• Linked Issue: Closes Invalid RUST_LOG value silently falls back to "info" with no warning logged #690

Verification Gate

I confirm the following commands were run and exited 0. (Paste the terminal output below):

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features --locked -- -D warnings
• RUSTFLAGS=-Dwarnings cargo test --all --locked
• cargo build --release --all --locked

Verification Output

$ cargo fmt --all -- --check
(exited 0)

$ cargo clippy --all-targets --all-features --locked -- -D warnings
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 1.43s
(exited 0)

$ RUSTFLAGS=-Dwarnings cargo test --all --locked
test result: ok. 282 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 2.23s
     Running unittests src/main.rs (target/debug/deps/oracle-4a320ca507af5aca.exe)
test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
(exited 0)

$ cargo build --release --all --locked
   Compiling oracle v0.1.0 (C:\Users\User\Documents\JOY\STELLAR\so4-oracle\oracle)
    Finished `release` profile [optimized] target(s) in 1.99s
(exited 0)

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