[Hackathon] Normalize empty/whitespace TESTSPRITE_PROFILE env var to fall back to the default profile

[0xshalah]

Discord Username / User ID: shalahuddin02

What does this improvement do?:
loadConfig() normalizes empty/whitespace-only TESTSPRITE_API_URL and
TESTSPRITE_API_KEY via normalizeEnvVar() so a blank env var is treated as
unset and falls through the ?? chain. TESTSPRITE_PROFILE was missed.

A blank TESTSPRITE_PROFILE= in a shell profile currently breaks every command
with a confusing VALIDATION_ERROR because the empty string fails the INI
section-name guard. This fix applies the same normalization so a blank profile
env var resolves to "default" — consistent with the other two env vars.
Details / implementation notes:
Fix: one-line change in src/lib/config.ts — wraps env.TESTSPRITE_PROFILE with
normalizeEnvVar(), matching the existing pattern for TESTSPRITE_API_URL and
TESTSPRITE_API_KEY.

Added 3 unit tests in src/lib/config.test.ts:

• empty TESTSPRITE_PROFILE → default
• whitespace-only TESTSPRITE_PROFILE → default
• blank TESTSPRITE_PROFILE reads credentials from the default profile

Local verification: eslint clean, prettier clean, tsc --noEmit clean,
14/14 config tests passing, no regressions in full suite.

PR will follow once this issue is triaged and assigned.

• ☑ I have searched existing issues and this is not a duplicate.
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