add macro guard around new test case for specific builds

[JacobBarthelmeh]

Fixes:

• nightly QT test
• FIPS v6 test with --enable-sp

[Copilot]

Pull request overview

This pull request adjusts the compile-time guards for the ECC shared-secret size-bounds API test so it is excluded from specific build configurations (notably certain WOLFSSL_SP_521 + FIPS/selftest combinations), preventing incompatible test compilation/execution in those environments.

Changes:

• Tightened the preprocessor conditions around ecc_shared_secret_size_bound() and test_wc_ecc_shared_secret_size_bounds() to additionally gate on WOLFSSL_SP_521 in combination with FIPS version and HAVE_SELFTEST.
• Kept the helper function and its corresponding test under matching guards to avoid unused/undefined references.

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[github-actions]

MemBrowse Memory Report

No memory changes detected for:

• gcc-arm-cortex-m0plus
• gcc-arm-cortex-m3
• gcc-arm-cortex-m4
• gcc-arm-cortex-m4-baremetal
• gcc-arm-cortex-m4-crypto-only
• gcc-arm-cortex-m4-dtls13
• gcc-arm-cortex-m4-min-ecc
• gcc-arm-cortex-m4-openssl-compat
• gcc-arm-cortex-m4-pkcs7
• gcc-arm-cortex-m4-pq
• gcc-arm-cortex-m4-rsa-only
• gcc-arm-cortex-m4-sp-math
• gcc-arm-cortex-m4-tls12
• gcc-arm-cortex-m4-tls13
• gcc-arm-cortex-m7
• gcc-arm-cortex-m7-pq
• gcc-arm-cortex-m7-tls13
• linuxkm-pie
• linuxkm-standard
• stm32-sim-stm32h753

[JacobBarthelmeh]

Retest this please Jenkins. FIPS 140-2 fail case.