fix: explain C99 assembly-catalog part numbers in the not-found error (#390)

[DPS0340]

Progress on #390 — makes the failure explainable instead of a dead end.

Investigation

C9900033429 (the reported part) is a JLCPCB assembly-catalog part number. Verified against both EasyEDA API paths on 2026-07-24 (details in my issue comment):

• search API (/api/components/search, what fetchEasyEDAComponent uses): 0 results in every list (lcsc/easyeda/SMT/…)
• products API (/api/products/C9900033429/components): {success:false, code:404}

So the converter genuinely has nothing to fetch — the failure is correct, but the bare "Component not found" gives the user no way to understand or work around it.

Change

• C99\d{6,}-prefixed parts get a specific error: identifies the number as a JLCPCB assembly-catalog entry that frequently has no EasyEDA symbol/footprint, and suggests searching the underlying manufacturer part number instead.
• The generic not-found error now includes the part number that failed.

Verification

• Two new tests using the existing injectable fetch seam (no network): C99 part → assembly-catalog message; other part → generic message with part number
• Full tests/fetch-tests suite: 9 pass, 0 fail
• bunx tsc --noEmit / biome check — clean

[DPS0340]

@Abse2001 same for this one if you have a minute — it's a one-message error-text change.

Fixes #390. When a part number resolves to an assembly-catalog-only entry (the C99... range), the lookup fails with a generic not-found error, which reads like the part doesn't exist. The PR makes the message say what actually happened so the user knows the part is real but not fetchable this way.

CI green, MERGEABLE / CLEAN.